I don't see why you need more than 6 air nymphs to generate the necessary amount of unstable gasses...so you should ditch the water nymph idea for using nymph tear's on air pillars from the beginning.
If it's slow, it's a bit difficult to make it faster, so the best option is to add more control/stall, e.g. Freezes, Squids, Shockwaves, Ice Shield, Fog Shield, Wings, Toadfish, Fire Nymphs, Rage Potions, Firebolts, Rain of Fire, Fire Shield, Owl's Eye. The problem, of course, is that even if each enemy creatures gets in 1 attack before dying, you can still die before pulling off the combo.