I had posted this before the site went down and it seemed to help some people so figured I'd move it over here:

If any of these help please comment, and if you have any helpful advice of your own please post to try and help others.
Anyway it's not much but here are a few things;

Thieves Forest:
If your suffering a few to many stray arrows early in the level try this; when you encounter the first two groups of thieves you'll notice a pile of yellow hay in top left corner, if you position yourself to the left of this hay and as far up as possible it will block all arrows coming from the other side allowing you to concentrate on the guys who follow you up there. It's a good corner to trap your foes in as well.

Pipistrello's (think that's how you spell it) Cave:
Remember you can block his tongue, it does 999 damage so yeah avoid it.

There is also an easy glitch to exploit here, Pipistrello has to hit someone efore he will return to the air. Once he emerges back up to the edge of the screen and hold block, once his tongue hits your sheild attack. You will have time for thre R2+triangle attacks or one full triangle melee attack then return to blocking to avoid his tongue. Repeat and he dies pretty quick and you don't have to deal with the little bats.

Two-player or more strategy for Lava World and Industrial Castle:
This worked really well for me on both levels. First have whoever has the most agility equip any weapons or animal orbs that can increase it further. Now when facing your enemies move both players to the bottom of the screen and into a corner to protect your back, have one player stand slightly in front and hold block while the other stands behind and unleashes arrows and magic as quickly as they can. The only two things to note here is first enemies who roll up on you can sometimes get over the blocker, but this is rare a they usually get caught with an arrow first and second bombs...... bombs are bad they can be blocked if they hit the shield straight on but if they go high be ready to evade or regroup after the hit.

Animal Orbs:
Do not under estimate the value of Rammy, yes as far as damage goes he sucks, but he can be incredibly useful in pushing enemies back to give you a little breathing room, especially if your using arrows.
Monkeyface is great to have in a multiplayer party as he increases the chances of food and valuables to be dropped, team him with Hawkster and you may be able to save yourself a potion or two in some areas.

Industrial Castle:
Having trouble with the falling spikes just past the acid shooter? Use a sandwich and run straight through, you will take one hit, but it's better than getting stuck in them. Just don't try to run through the electricity.

The elevator is not really as difficult as it seems, divide your party between the top and bottom to divide the enemies and pick them off with arrows or magic.
If your going solo try to draw the enemies to the bottom of the elevator spam your magic then move to the top opposite corner and repeat keep it up and they should fall pretty quick (well quick by insane standards)

Desert (1st area):
This area is easier with less people, because the more in your party the more scarabs you face. With 2 in your party you face 1-3 scarabs at a time and only 3 once. With 3 or 4 in your party you will face 4 scarabs at least twice, this is bad.

Sacrab tactics:
The most two most annoying things with the sacrabs are the underground attack and the fact that scorpions keep popping on the screen as your trying to fight them, sometimes hitting your from behind. So for the first one remember you can block the underground attack, and if there are more than one person in your party the sacrabs will focus on only one of you at a time. This means instead of jumping around each other and getting yourselves killed you should seperate find who is being targeted by each sacrab and block accordingly. For the second thing station yourself at either the very top or very bottom of the screen and the scorpions will not spawn on your line leaving you free to concentrate on the scarabs.

Desrt Castle Rooftops:
Remeber the Tornado attacks are considered physical and thus can be blocked with your sheild. It is easier to block and wait for them to finish sometimes, rather than try to hit them out of it.

Remember you can skip severial fights here; avoid some of the skeletons by leaving the headstones intact and avoid the group of forest knights by circling around the back of the houses, just before the troll, and stay to the right of the screen while fighting. If you walk over by the houses to pick up any items it will trigger the wave you just avoided.

Corn Boss:
2 quick strategies for dealing with one of the most annoying bosses in insane mode.
-First is the straight line idea. Once Corn Boss begins jumping around make your way to the far side of the level. When he resubmerges run in a straight line across the screen, when he pops up behind you turn around and fire magic or arrows. When he sinks again simply run the other direction and repeat.
Note: If playing with multipal players just make sure you are on different elevations and don't cross each others lines of travel to avoid the Boss popping up behind one player and striking another.
-Second is the strong man idea. This requires you to have some extra potions and sandwiches to pull off. Once he starts moving eat a sandwich and stand still. He can't knock over you in brute mode, so when he pops up you'll take about 300 damage, depending on your defense, then go nuts attacking. Watch your health and sandwich countdown then just repeat.
Note: This strategy is much harder to use in multiplayer games, unless everyone stays close and uses the same idea.

Ice Castle:
The Ice Castle is kind of a joke level for insane mode. Equip Yeti and you can just hold block to walk down the path with arrows and the Ice King's freeze shot, you will take "9" damage from each ice shot but if your level is decent this will not matter. Defeat the 3 icekimoes and the Ice King just a simple matter of grinding him down (He has an obscene amount of life).

Take-off point:
I think most people know this one already but I'll add it for anyone who doesn't as your flying through the sky watch the top of the screen, just before a big boulder drops a few tiny rocks appear at the top of the screen to let you know where its going to appear.

Wizard's Castle:
Cult Minions are completely IMMUNE to elemental magic and are NOT pushed back by arrows, so be ready to get physical. If you stay near to top of the screen you will notice slight difference in the depth around furniture and the pillars, these can be used to block magic attack but you can still strike around them, example; with the first set of four minions go to the right of the pillar at the top of the screen and face left, then spam square through the pillar.

Necromancer Solo Run:
Okay so I'm a narcissist and I'm posting this mostly to brag that I soloed the Necromancer, so sue me. First off I DO NOT reccomend doing this solo, it took me a good 30 tries at least to pull this off and find something that worked. Just get some friends to help you do it, but if your gonna go for it hopfully this will help.
Wave 1: 2 options first if your good with the airial combos get up and stay up, keep your opponents in the air and try to transistion from man-to-man. second, use the air combos to move around and spam magic "R2+triangle" whenever you hit the ground
Wave 2: This is when you will want to quit the game and never return, get to the bottom right corner of the room, hold block and back in and down as far as you can (this will make it hard for the big guys to grab or hit you, I don't know why but it does I got struck once and never got grabbed). Now alternate blocking whenever you see projectiles coming and spamming arrows when they are not. If anyone tries to come near you spam your triangle magic to push them back. Watch out for roll up attacks and the occasionial bomb.
Necromancer: Congrats if you beat the 2nd wave your pretty much done, the Necro does about the same damage he did in regular mode and can really just be pounded away pretty quick, just keep an eye on you life as you don't want to do this twice. Okay good luck, if you pull it off post your brag here.

Dark Wizard:
When the wizard is in his red/ blue bubble form your R2+X attack counts as both a physical and magic attack and hurts him in both forms. Not the most effective attack, but it can be helpful in some situations.

Industrialist = Medic
If you have an industrialist in your party you may find them to be most useful in the role of medic, the reason for this is once their magic is close to maxed out the blades spawn for almost 2 secs using the "R2+triangle". If one of your allies falls have the industrialist run over to the body and quickly spam magic in both directions this should give you enough time to do a rapid revive. (Note: you still need to watch out for ranged attacks, and don't do this in the desert surrounded by scarabs)

Alright that's all I got thanks to everyone who left feedback and good luck to all who are still working through.
Hopefully this will help, good luck.

I am willing to help anyone who ask, however if you act like an ass your on your own.

If your looking to get through Insane mode a little faster you can team up with someone who has already beaten it and play the following levels.

Home Castle
Castle Keep (Get to do them both)
Barbarian Boss
Theives Forest
Abandon Mill
Pipistrello's Cave
Cyclops Fortress
Lava World
Industrial Castle
Dock & Pirate Ship (Got to do both)
Desert Chase
Sand Castle Rooftop
Medusa's Lair (There is a glitch that allows you to skip Marshland/ Corn Boss completely)
Ice Castle
Wizard Castle Take-off
Final Battle

After you beat the final battle the trophy will not appear because the game believes you have not cleared all the levels (and it's right). Simply exit your game and restart with the same character, all the levels except Full Moon will now be unlocked (Full Moon is the only level that can just be passed through even with an arrow over it). Return to the Final Battle and rebeat the Wizard, the trophy should pop upon his defeat.