Air (lightning) and Earth (poison) is not a good combination against the poison-infested undead.
Fire however, lots and lots of fire - is. The only good spell on earth good against undead is midnight oil (combined with fire)
The water spells are easily underestimated too but they CC like crazy - things are easily frozen. Doesn't synegise well with fire however. Best to stick to setting everything alight with fire. Hybrids are the best - don't let anyone tell you otherwise you just have to know the spell system.
Air is very unpredictable as lightning can backfire