There should be an option that is always available where you can go in to a very simple (quick loading) solo battle against a random PvAI or PvE enemy depending on where you set a slider for difficulty. Low difficulty is an easy PvE/AI enemy. Extreme difficulty gives you a boss type enemy or other heavy target.
It should also reward you with minor credits/synergy/resources depending on the diffucilty. Very weak PvE enemies like Rabids and Demomen should spawn in swarms.
Quicker easy battles = low reward
Longer hard battles = high reward
Reward should only be issued if you defeat your target. Loot should be max one spot.
Should also have a box to select “infinite” mode where as soon as you kill a target, another spawns randomly. Kills multiply reward just as if you did them one at a time. Infinite mode ends when you die or self-destruct.