Obviously it needs to be objectively worse then fighting people time wise. But something as simple as taking the inverse of the active time of howard multiplied by some base times its difficulty. So the quicker you win and putting him at max difficulty results in more coins. With a minimum of literally 1. But youd only get like 10 coins for beating him really fast on lv3

This also helps when waiting for longer que times

And would pair extremely well with the planed updates to howard to make him not a push over after 20 hours in the game