Depends on what you are grinding and at which point. Basically easiest low effort grinding spot is 1st mission to protect the village. Top and right spots are swordswomen spawns, bottom spot is archer spawn. You just take anyone with a range weapon to the bottom (archer or throwing axeman). You can buy all the cheap weapons+clovers and just occasionally heal waiting for enemies to kill themselves. They spawn till turn 100. After approximately level 22-24 it's not useful anymore as you gain 2-4 exp per kill, you better finish some story missions.
For lategame money grinding I'd suggest Alexandra bossfight. You use Robert/Albert and Warren with Devil Axe+ Throwing axe with a lot of HP and maxed skill+strength stats and let's be honest DEF as well. Sometimes she dies after turn 1 if you were lucky with CRIT RNG. You can optionally leave 4-5 HP Alexandra for some underleveled companions. This is by far the easiest milking spot for crazy ass money. You can earn like 13k every 20 seconds or so.
And lategame leveling is best done in the final dungeon, you just conveniently position your units in such a way they always have an upperhand in weapons triangle. But be wary there are Spearwomen with axebreakers and swordswomen with crazy-ass crit chance. This is by far the most level rewarding and the most dangerous grinding spot. From my observation they spawn until 30ish turn.
