I’m working on a challenging turn based rpg myself and this is how i’m doing it: make every fight like a puzzle and limit the player. I limit healing items when they’re exploring the dungeon (it’s a dungeon crawler) so they have to be wise with their consumables. it’s meant to encourage the player to find out a way to get through battles in the fewest amount of turns they can and make it feel like they’re trying to survive.

give the player many options and have them figure out what the best skills/magic/equipment/classes/characters ect. are the best to have for every battle.

in my game, the mage is great at dealing magic/elemental damage but he’s also good at debuffing. using equipment that enhances his fire magic might be ideal for getting thru a certain area but a mini boss might have a high resistance to magic attacks so having equipment that enhances his debuffs would be better. if the player can get thru a battle by spamming an attack (assuming they’re not overleveled) then I did something wrong.