One trooper, one sum, and a horizon full of arithmetic.
Every monster wears a digit. Shoot the answer into existence — ones column first, the way you'd write it: for 5 × 4 you shoot a 0, then a 2.
Need a digit nobody is carrying? Hold S to kneel and shoot two monsters instead — green fire fuses them into one digit, and pays double. You can't jump or kick while you're down there.
The ones carrying junk still walk at you. W to jump them, D to kick one back through the crowd. Anything that reaches you takes a life.
Clear enough problems and the shift stops for a coffee break: a stretch with nothing at all gone wrong pays a bonus and hands two lives back, and you pick whether to stay on this grade or take the next one up.