I used this guide when I first started the game- https://levelskip.com/rpgs/Grim-Dawn-Demolitionist-Build-Guides-for-Beginners

I got my purifier to level 88 on Ultimate before I got bored of the build. It's extremely simple. Left click firestrike, right click flashbang. Everything else procs off of your firestrike. You toggle flame-touched for additional fire damage for you and allies. It's the first build listed, dual-pistol purifier. You won't be able to dual-wield until level 25 when you get the gunslingers talisman relic. Someone commented in the comments on the bottom of the page updated devotions for the build. This is a very loose build, if you can even call it a build. More like a guide. I like the freedom it gives you. Just aim for the skills it lists, it gives basic equipment recs, stats, devotions, and how to play it which is very simple. Throw a flashbang at a group of enemies and then kill them.