I've been taking on Insanity with my Soldier at a slower pace than my first run through...my teammates really aggravate me. Even when I point them to specific cover spots they still somehow manage to get themselves killed off and I end up having to slowly take out the opposition by myself... I'm dreading getting to the parts with Scions and Praetorians.
You really gotta babysit teammates on Insane. They generally only can take about half the damage you can, a lot less if you have all the Shepard only upgrades. Regardless of difficulty, the thing that changes is enemy aggression, their weapon damage, and how much damage
they can take. Exceptions are Grunt, Jacob, and Legion, who have a bit better survival because of their specific upgrades/abilities. Praetorians aren't too hard, but Scions are nasty, especially in two parts of the game.
You really need to assemble your team for what you're going to be fighting on Insane. No more half-assing it by skipping out on biotics or engineer skills and going balls to the wall power Soldier/Zaeed/Grunt or other terrible for the situation group configurations. Barrier using enemies? You NEED Warp or Concussive shot, preferably Heavy versions of each. Heavy Mechs/Geth Primes? Overload, Overload, Overload. And don't be afraid to cheese skills like pull and throw comboing, or hacking synthetics and then running up and just meleeing them.
I'll tell you my Soldier build that's made Insane a lot more tolerable.
4/4 Hardened Adrenaline.
2/4 Concussive Shot
2/4 Disrupter Ammo
2/4 Incendiary Ammo
4/4 Improved Cryo Ammo
4/4 Shock Trooper
4/4 Geth Shielding, evolved to the one that gives +10% weapon damage.
At the start of each mission:
Sniper Rifle, set Incendiary Ammo.
Shotgun, set Disrupter Ammo.
Assault Rifle, set Cryo Ammo.
Cryo Ammo is a beast in Insane. It lasts it's full duration on most enemies, including minibosses like Heavy Mechs. Once an enemy is frozen, you can start hammering on another while you tell your teammates to finish off the one encased in ice. While useless against Barriers, Shields, and Armor, it basically negates health bars entirely. Improved Cryo Ammo almost always freezes in a single hit.
The only enemies I've seen come out of being frozen quickly are Vanguard class enemies. And always set cover to prevent flanking when applicable.