Tryndamere's ult should be a passive
It works exactly the same as it does now, but it activates upon hitting one health rather than requiring manual activation. Although this will be a buff in any fight Tryndamere would ordinarily win, this is a nerf to people outside of dumpster elo. Tryndamere is usually dealt with by creating situations that are massively unwinnable for him, and in order to keep forcing these situations, he can save his ult. With his ult up, he requires 2-4 people to deal with him, but with it down a single person can at least keep him from backdooring since he won't have the safety net to towerdive.
If necessary, give his ultimate an increased cooldown of 10-15 seconds so that it will further solidify this weakness. This change would remove some of the skillfloor to playing him but it would also make him easier to deal with in some situations.