Need help with Fighter build

Huh? Doesn't Ground Smash hit 3 monsters for 300% damage? So that would hit a total of 900% damage, while Slash Blast is 180% on 6 monsters. Slash Blast seems to only outdamage Ground Smash on mobs of 6.

Hit level 48, maxed Ground Smash has replaced both Power Strike and Slashblast for me. Currently maxing Rage over Final Attack, is that a good idea?

I'm really used to the heavy utility of Slash Blast being able to hit, knockback, and kill more monsters at once, which is a heavy benefit.

On four monsters, you can Ground Smash them x amount of times, but the last one you'll have to continue Ground Smashing x amount of times again. Alternatively, you could just Slash Blast them for fewer total hits to finish all four. It does depend a lot on where you're training and how strong you are, but people should look at a skill beyond its %/s.

Max Final Attack, in theory, adds an extra 60% per hit on average, but it's probability based. Rage is constant. It's really up to preference, but I do believe that a Final Attack build, or even a Fundamentals build would be better in terms of training, especially if you can rely on something like Warrior Pills in the mean time.