first the grenades should be thrown into the leg of kickback boss (3 grenades ) then shot his leg dont shot any other part of his body. Probably i kill boss by shooting his leg without any problems but with grenades it will be easier. The paid booster give you only 2 slot which is not enought. So its better to have explosive expert
I never throw any more than one grenade on kickback enemies. That's enough to break the armor from their legs, so you can finish them off.
TS is by far the best booster in co-op. I would give up any other two boosters for TS by itself.
it really isn't needed if you're playing with people who know what they are doing, and the fact that the NPCs aren't as grenade dependent as the mainstream uncharted player is now ... (with the exception that if you're using the cluster than TS is mandatory!!)