Kai Leng was an extremely easy boss on insanity for both my Vanguard and my Infiltrator.
Infiltrator: Black Widow + Tac Clock -> Scoot around on the outskirts and snipe the reinforcements -> shoot Kai Leng. Phatoms are easy. Just Shield Drain + shot anywhere on the body IMMEDIATELY after -> headshot while she's stunned and dead phantom in 2 shots.
Vanguard: Charge -> roll -> Half Nova -> FALCOOOON PAUNCH! -> Charge and repeat until dead. The only time I noticed the reinforcements at all where the Phantoms. I Charged -> Nova'd -> FALCOOOOON PAUNCH! = dead phantom. If the two were close enough for my Area Charge and Nova to hit both... cool. Wraith to face = dead space ninja keke

. This was by far the easiest out of the two classes I used to beat insanity. I never took damage to my health at all. My infiltrator lost a bar.
Sounds like you just need to turn down the difficulty bro. Take EDI and user her Decoy if you *really* are having difficulty and you don't want to turn down the difficulty.