To enable the AoE's, in H_Extra.lua (open in notepad, apparently the GUI doesn't save changes correctly), set the Use(homun type)(skill name) = 1 instead of =0 and they'll work without further modification. Tweak AutoMobCount to adjust how many monsters it needs to see before it'll use an antimob skill.
Lava Slide maxes out at about 4.x k vs neutral monsters with pyro up. So youre talking 7-8k vs scarabs, 8-9 to 10-11 with flitting (depending on lvl), 11-12 with bloodlust... Not half bad, really.
I don't think scarabs is the ideal way to lvl though - Remember that homuns never get a level adjustment - scarabs are interesting mainly because they're in range of 150.