Not sure if you're already aware but the 2 corners (marked X) work as safespots also, so you can then just flick melee + 1 other mob rather than all 3.
Stand on SE corner will safespot eastern side and vice versa.
Another thing you could do is run towards eastern pillar (blocking the ranger), freezing the melee as you run then take him down.
Easy to say in hindsight but anyway hopefully it offers you some ideas for next time
Very solid first few attempts!