Grav/Storm, of course. Both grav and storm bring decent damage early on. By 16 you have Lift, Crush, Propel, Crushing Field, Gale, and Freezing Rain as decent damage.
Plant/Dark is good, as well. Dark is defensive, with a good heal. Plant has Seeds of Confusion at level 8.
Fire, of course, is one everyone recommends for good damage. I am not fond of the playstyle, but it is good at damage.
Remember that playing solo, it is better to consider Debuffs than Buffs for your secondary. -ToHit & -Damage helps you survive, -Def, -Res helps you speed up your kill rate.