problem is, on the hard campaign, I'm stuck on the wizards.
Yeah I got stuck on that stage too. It is that stage that challenges most people (at least hard / Insane Mode) because the Magikill have explosions, electricity and poison that can kill off your troops fast. There are various ways to beat it. I just beat it today on Insane mode in one attempt... The following are recommendations:
You can build an army of speartons and archidons 50/50 but there is one caveat. Controlling the troops. The speartons must be controlled so they do not get too damaged by the magikill's magic or spells. If they get poised, dont forget to garrison them to take the spell off and cure them. If you use archidons make sure that you control them too because otherwise one explosion will send them to the ground.
Another strategy is to build an army of Shadowrath and swarmed them around the battlefield until all the magikill, merics, and their miners are dead. I've seen this strategy employed by other people. Its all up to you though. Whatever works best for you.