Pyromancer is all about the Burning debuff. Burning affects you and enemies permanently, but while enemies take damage from burn, you'll heal from it instead. The power of the burn is based on the intelligence stat when afflicted, and once you afflict it at the start of battle, it will be based on the highest intelligence stat among your creatures. Burning enemies can't resurrect, you gain a buff that prevents the next attack or spell damage when ever your Burn affects your creatures, enemies get a Bomb on being burned, your creatures can resurrect if killed while burning, spells that interact with Burning don't use charges, giving you unlimited casting with those spells. You'll eventually unlock creatures that can boost the Burning potency to absurd levels. Late game specs where enemies start their first turn then immediately die due to the power of the Burning debuff are pretty common.

Early game you probably won't see too much that interacts with burning, so I'd pick defensive creatures at first, and wait for the realm where the Efreet race is at. Efreets are all about the power of Burning. Several of them do things that your own perks do, so if you grab those, make sure to respec at the wardrobe (you can do this for free any time you're in your castle) so that you aren't putting points into a perk that isn't useful. Efreet do things like permanent burning, healing from burning, if an enemy takes burning damage, all enemies take 30% of that damage, and provoking causes enemy burning potency to be increased by 30% (which stacks multiplicatively). The last one is one of the big ones for late game Pyro builds.

Another good choice is the Magma Golem. I'm not sure when it unlocks, but basically you want that one provoking, to force enemies to attack it. If it's attacked, enemies are afflicted with burning. If they already have burning, the potency is increased 30%. Between that and provoking causing burning potency to increase, you can scale the enemy team's burning quickly.

Hell Knight is very straightforward for a new player. It's basically all about pushing the power of your attacks as far as possible. Eventually you end up with all your creatures getting an extra attack, all attacks hitting the entire team, you always crit, enemies can't dodge, and your team is immune to Scorned, the debuff that disables attacking. The main downside to this one is a complete lack of defensive options, so you'll need to choose some of your creatures to offset this downside, or else lean into the glass cannony nature of the spec and push for getting quick kills before the enemy has a chance to retaliate.