Hey, I just posted a whole thing about the demolitionist class. Forum Link
Of course this was more about playing the demolitionist without a second mastery, but this is my favorite class so here’s some advice.
Since it looks like you’re most interested in the skills of the demolitionist then know that it’s entirely possible to solo the class through the entire game’s content. And this way all your skill points go into bombs.
If you want to use all the bombs, then go for a barrelsmith build like this: Grim Tools. It’s a completely viable build for endgame content with only moderate farming required. You’ll need the guns to combine for 100% fire conversion, and the two monster infrequents will take some farming, but it’s doable.
There’s also the more unconventional option of going cold damage, like this: Grim Tools. It uses the same spells as before, they’re just not as powerful.
But really, the fun in Grim Dawn comes from the dual class system, so I get wanting to go that route. The problem is that it necessitates a wider spread in skill points so you have to be more selective about what spells you go all in on. So at most you can generally only use one or two bombs as a primary attack. The good news is that every spell in the demo’s arsenal is viable if maxed out with supporting gear enhancements. Even thermite mine, mortar trap, and vindictive flame can all be your primary damage dealers if built around them. So there’s some flexibility with build options. If you think something sounds fun, then try it.
For what it’s worth, here are some builds that I’ve played around with over the years.
Elementalist - Grim Tools - Along the lines of what you were describing before. It has lots of bombs and other spells to dish out damage. However, I abandoned this build because it’s a real keyboard smasher. It was fun at first, but I grew tired of it after a while. If, on the other hand, you want to cast lots of lightning damage to deal lots of hurt, this could work. And once again the guns will need to get as close to 100% fire conversion for this to fully work.
Commando - Grim Tools - Fairly viable. The Mythical Beronath, Reforged works wonders for the demolitionist because almost everything is elemental and chaos damage. So this one item converts almost all damage to physical making builds like this easier to create.
Saboteur - Grim Tools - Another fairly viable option. Rimetounge enhanced blade trap so I invested in it, but it’s really only useful for mobs and not bosses. Still, it works. At least in the main game. Of course this build requires items like Alkamos’ Warsword to convert damage to cold in the early game, so I wouldn’t recommend it on a first playthrough.
Pyromancer - Grim Tools - Only tested a little bit, and that was many patches ago. Was an idea to use blackwater cocktail and thermite mine as primary damage dealers which led to the using the occultist class for extra chaos damage. Sigil of consumption is there for extra damage and life steal. It should deal adequate damage to bring mobs down, but I’m not sure about it’s long term survivability.
Purifier - Grim Tools - Only tested this one briefly, as I made it in middle of completing my solo demolitionist roster. So I can’t say anything about its long term viability. It should work, at least in theory.
There’s a lot of choices with builds in this game. Most things are viable for the main campaign if you just build around them. So go with whatever sound like fun to you.