For last level I just did front four rows spikes, back three rows archers, then knights in the final fillable row. Shotgun on the horizon, and lightning whenever a dragon, fire elemental, or giant ogre got halfway through the spikes.
I remember giving up after barely not beating the last level a million times in a row a year or two ago. I think it's about time to try again, if only briefly -- this might be my favorite game on Kongregate (certainly on the Citadel at least).
 "Siege Knight"){kind=icon}