I completed Insane mode a week ago or so and died roughly three times. One of them was in Chapter 2 which made me pretty pissed, and one by the Kayran, but I still carried on.
Also, since you've completed the game on all other difficulties, you're probably already pretty experienced, but I'm sure you'll find something useful.
To all those less experienced with the game, my advice should help you a lot
Build
The Build I was using was pretty much damage/health based. I spent around 8 points in the Alchemy tree, so that I could get the ability that increased my mutagen strength by 30% (Don't use your Power Mutagens or any mutagens at all before maxing this ability).
I spent 3 or 4 points in the Magic tree so that I could get my Quen to level 2 which makes it deflect damage and spread it among others. The rest I spent in Swordsmanship trying to build up my damage and health.
The most important tip is probably to max the ability that decreases the damage you take from backstabs as early as possible, which should be at level 9.
Always use potions, oils and bombs!
Seriously, they help a lot. Bombs can be very useful against large packs of enemies, oils help kill creatures more quickly, and potions are critical lifesavers.
My favorite bomb selection: Grapeshot, Dragon's Dream.
My favorite oil: Whetstone.
My favorite potion combo: Swallow, Rook, and Tawny Owl/Golden Oriole.
Monsters
When fighting Nekkers with your sword, have Quen up ALL the time and attack from a distance so you can use the three-hit combo attack or they will just dodge your attacks. As soon as you lose the Quen you have to retreat and put it on again or you'll most likely get one hitted by a Nekker warrior.
But the most powerful and easiest tactic against them is using Grapeshot bombs, which you should carry all the time.
And for the Endregas, they are pretty easy to kill. Just pop Quen, and jump in and do some strong attacks (2 or 3 attacks should kill one) and dodge all their attacks. As for the Endrega Queens, wait for it to charge, dodge it and backstab it with strong attacks. It should die pretty easily.
After you've passed the Kayran and Chapter 1 it actually gets pretty easy. But you should watch out for the Harpies, they have a nasty bleeding effect that killed me before I could use Quen to remove it. And the Rotfiends aren't to toy around with either. Their charge is deadly, and the explosion when they die is pretty devastating as well.
Oh, and be very, very careful at the Dragon fight in Chapter 3. With a level 35 Geralt with the best equipment, a lot of Vitality, Armor, and Damage Reduction, it still took 60% of my HP with one hit.
Armor/Weapons
The armor/weapons I used when I played Insane were the following:
Chapter 1
Raven Armor
This armor has excellent stats, and is probably the best armor to use until you can craft the Kayran Carapace armor. If you didn't play Witcher 1 however and you don't have it, use the Blue Stripes armor and later replace it with Hunter's Armor + some Vitality upgrades once you can.
Robust Cutlass/Sword of Kaer Morhen
The Robust Cutlass can be found in the haunted hospital in a chest. Best taken early on, but the Wraiths can be really tough so beware.
Sword of Kaer Morhen can only be found pretty late on in the chapter, so I recommend crafting/buying a Robust Witcher's Sword until then. After you've talked with Iorveth where the Arachas fight was, the sword is found on a pile of bones outside the hideout cave.
Chapter 2
Kayran Carapace Armor
This armor is excellent and can serve you well during the entire chapter. In my opinion it's better than the Armor of Ban Ard since it gives a lot more Resistance. There are better armors however, such as the Draug Armor. And there is the Dragon Scale Armor, but it can only be obtained on Iorveth's path.
Gwyhyr/Robust Meteorite Sword
Gwyhyr is obtained only on Iorveth's path by completing the quest Balitmore's Nightmare, but the Sword of Ban Ard which isn't as good as Gwyhyr but still good, can be obtained by both. It is located in the crypts near Vergen inside the chest where the Banner of the Barn Ard is located. The Robust Meteorite Sword's can be bought/crafted at any time.
Chapter 3
Vran Armor + Elder Blood Gauntlets/Trousers/Boots
In order to get the Vran armor you must have completed the Mystic River quest, which you get by exploring all the ships in the game. The Elder Blood gear you get to craft during the Gargoyle quest.
Forgotten Vran Sword/Sword of Addan Deith
The Forgotten Vran Sword is in my opinion better than Caerme because of the extra rune slot. The Sword of Addan Deith is the best silver sword for those following the swordsmanpath.
That's pretty much all I can come up with
