Not easy to beat a bombot in normal explore speed-wise, if you want only traitstones, it’s probably the most optimal (as long as you have the Mechanist traited).
Personally if I do explore (and I rarely do, too lazy) - I try to get some free souls on the side too. So my go-to team for explore is double-valk + mercy + either Mab or Bat. Mab is faster, with a good board you win in “1 turn” (popular thing recently 
), because you go mercy → valk → mab → valk → mab and collect your traitstone and get close to the cap souls too. The downside is ofc rolling something Impervious to play against (you can just exit and start another one).
The Crimson Bat version of the team doesn’t have that problem, but also doesn’t have a chance for the extra turn with enough blue around. Either works really, personal preference.
e/ since I didn’t mention it before, for both teams I use double yellow banner (so that mercy can charge both valks, since valk doesn’t have much problem charging the bat, and worst case - bat gets blue mana and red mana priority too).
and the order I use is Bat(or mab) / mercy / valk / valk (why mab exposed? why not? in normal explore there are no threats, and if mab is first, you can put mercy 2nd which saves you a second on waiting to use her - the traits triggering, so it’s just about min-maxing).