This is kind of a tough build to play. It's safe if you don't ever see the monsters, but thats kind of boring imho. Spam glacial spike on your merc in mobs to keep him alive longer, and a reapers toll would do wonders if you can get your paws on one. I know I can't.
To get hydra to hit as you tele you need to move in 1 dimension. Lay hydras at H and run/tele between A and B. This works well with FO too, and with practice you can actually sweet spot monsters on the move pretty consistently.
When I run baal with 8 players it doesn't hit all that hard. It's definitely not a powerhouse. I've found that if i group my hydras together and away from the group of enemies they will pick on a single target. Except on the council member boss pack, I cast on top of party members to scare the crap out of them.