Also, I feel you’re expecting a Pyromancer in GD to be the same as in other games. Here’s a description of the Demolitionist mastery:
In the same way that Crate didn’t want staves for magic users, Demolitionist is something different from the classic/standard fire user. With Blackwater Cocktail alone maxed you can pretty much wipe out most trash mobs; add in the other options like Canister Bomb, Grenado and Mortar Trap and you have plenty of firey options to play with. Thermite Mines as well can do a lot of damage. Then add in procs from various devotions like Imp, Solael’s Witchblade, Magi and Torch and you’ll have fire all over the screen.
Here are a couple of endgame builds focusing on fire.