Summoning is too expensive anyway. I agree.
(Sorry that I dont know spell names because my game is in german) When first reaching this i used poison eater because the debuff works this way: when you damage him you get it and the next time you end your turn the debuff gets removed and you loose spirit. Poison lets you stack spirit (+1 each time) and doesnt give your character the debuff. Then i used a tank and the free SR with high damage if he survived 300 tu to deal a lot of damage fast. If you have the other free SR you could also let the boss kill one of your guys on purpose and use the SR’s revenge spell. If your still characters dont deal enough damage just play the online events to get ingredients which let you ultra evolve them. You can get ssr weapons or like currently a SR character if you just use your mana for the event once a day. Also if you feel like your units should deal more dmg you should check the following: Did you level up weapons yet and equip the best ones? Did you equip the best accessories you have? Did you awaken duplicate characters?
That boss is definetely killable f2p and not really a powerwall to make you spend
Try using heries that has +2Spirits on their normal attack.