What would a mode where everyone has 75% damage reduction look like?

OP champions (if you can even call them OP in such a mode) would include champions who primarily use basic attacks as their main form of damage, don't use mana as a resource, or have abilities with low cooldowns and/or high base damage. UP champions would be everyone else.

Due to the 75% damage reduction, you would pretty much have to do enough damage to kill someone at least 4 times over. Champions who use mana will run out of mana trying to deal damage with abilities. Champions with low cooldown abilities can dish out their damage more often in an extended time frame. Champions with high base damage abilities won't have to expend as much resource as champions with lower base damage abilities to deal the same amount of damage.

A wet noodle fight.

League of Tanks and ADCs

Any resourceless champion would be good.