after you finish the campaign if you want many credits then u grind in campaign.
that seems boring so add something like survival battles.
it is like this : you use 1 mech to fight infinite amount of mechs, the mechs first are weakest ones, then the next mech will be stronger, etc. after u win against a mech, then your mech will only be recovered the amount of 20% of max hp/energy/heat/regeneration/cooling (if damaged), and you got 30% of max bullets/rockets.
the rewards are based on how much battles u survive.

I HIGHTLY APROVE THIS!