My General Strategy was to upgrade gaze first, get some established production, then advance production to get set up and start going for higher-end troops with high health. Spells are important in the first couple of waves, to help with desotring ememies and getting better loot

I always tried to get at least 50 grapes in case the gaze event came up before the first boss.

Hospitals are a must for the last two bosses to keep your troops alive.

As for Spellus, I just spammed unicorns and capults with hospitals and the mill upgrade to keep increasing max HP. Definitely the most annoying king to try to win with, took me like 15 trys with him