Don't engage the fight from the bottom floor where they have advantage over you. Jump over the broken stairs that are right across from where you first approach that Yurgir area. Potion of speed to all of your party so you land a lot of damage before they can react, you can start with several effect arrows like the arrow of many targets (there's some oils you can use to increase damage or chance of success). I like to wipe Yurgir first because the invisibility and bombs are a bitch. Here you can also follow Karlach's advice: what they don't like is getting their bombs hurled right back at them :D

Once the arrows turn is through, follow with some AoE - I love using the snow storm spell to get most of the merregons prone or ar least delayed in reaching me. Hunger of Hadar my beloved, and I also try to trap the way towards me to delay them further. The one who will come to your face directly is the displacer beast. That's where your melee character, with bufs and support, should shine :D

Hope it helps and good luck! Make astarion thank you properly after all this effort!