i dont understand your issue, you just run into them and beat the crap out of them? you even have allies wich tank for you. i used all my resources to get in faster, so none of the NPC dies. and it aint a problem whatsoever.
I generally agree, that you cant "math out" every encounter. But this is for sure the game where it is the easiest to get those numbers. you basicly always have advantage for free, hit rates are so insanely high, that bad RNG comes into account very rarely in fights, as the diviation is lowered by the rerolls you get thrown at with like confetti.
Did i sometimes not go melee? ofcause.
for example if you finished the 3 bosses in the goblin dungeon, you will have to clear the entire goblin camp.
So you just go upstairs murder the sleeping ones, set a fire at the stairs and shot -> move in cover. move out of cover -> shoot -> move into cover. Obviously astarion can not go down and tank all 20 of them, thats not gonna work. But those encounters rarely happen. And in those encounters he still out DPSes rangers, because he is upstars -> advantage -> sneak attack -> higher damage. Heck even DEX9 shadowheart outdamages my ranger, as she is throwing oil and alchemist fires instead of using a bow. Bows are a trap in this game. never forget, that any giving character, regardless of skills and abilities, is still an alchemist in that game.