- You can hire mercenaries into slots which appear at the bottom of your screen and open up as the level progresses. It doesn't make it massively clear when that limit increases, so keep checking. Having a numerical advantage is always a good thing, but it's massively important at the start of fights because it helps you zone areas of the map and puts the opponent on a back foot from the off.
- Also with mercenaries, they last for as many fights as they survive, while summons don't. So if you've a choice between sacrificing one or the other, always go for the summon. Because mercenaries can and will die it's sometimes worth leaving a good mercenary unhired so you can grab them later for more important fights.
- Mounts are the only thing in the game that give your wizard an extra hit point, so they're generally a good idea, however note that you can't collect (or even move onto) mana or treasure chests when mounted. Since mounting/dismounting takes a turn each way, you might want to hold off saddling up if there's easy mana or money around to collect.
- Beware Magic Bolts - don't get too close to enemy wizards because they might just one-shot you and win a game you were otherwise well placed in.
- Area control like Blobs, Woods and Tangle Vine are great for pacing the game, especially if you need to go defensive but also are surprisingly useful on the offense if you can pin an opponent into a corner.
- Quantity beats quality. That's probably just play style, but to me the game seems weighted towards quick wins such that taking time to try and get 'bigger' units on the fields doesn't work.