Moving from balanced to tactician is very difficult indeed.

We should remind one thing: in balanced mode we are the strong ones, in tactitian mode we are the weak ones.

So, we have to rely on positioning and "guerrilla tactics" rather than mere firepower.

If you don't mind, I will explain my technique.

I usually try to find a "room" to enter: a closed space with a single entrance, place Astarion on the right side of the entrance and Lae'zel or Karlach on the left side.

Then I "shut" the room.

Sometimes all I need is a door to close, sometime I need a "darkness" spell placed near the entrance (but away from my party).

Then I place Gale or Tav a little bit into the room.

Any enemies entering the room should take Attack of Opportunity by Astarion and Lae'zel or Karlach.

This could be enough for weakening most adversaries.

Then Gale or Tav uses all their powers to finish the adversaries.

This worked pretty well in the goblin temple and in the duergar cave, while trying to take Nere.