I'm going to make this short:
The best way to get levels in this game is actually quit easy. First, make sure you equipt good "grinding genes". these include any genes which boost your stats by an x number of point when you level up.
This is the easy part. Pick a map of your choice and pick the level 10 levels above your current level. So if you're level 19, try to pick the lvl 30-40 map, if at all possible. If you're level 31, pick the level 40-50 map, a so on.
Make sure you SAVE AFTER EVERY BATTLE. You never know when you could get monster assualted and lose some money.