Sometimes, it completely depends on the game. I only played Awakening and Shadows of Valentia, so I had a bit of experience. In both, you have for example, in Shadows of Valentia Terrors reappear to battle again for grinding. Also, Awakening has side-quest recruiting battles, and, depending on the level of the enemy, they are used for grinding. As well as the ambush encounters, too, so most people I watched on YouTube or fellow FE players said that playing DLC maps and side quests helped with the whole support shtick. Hope this helped!
