Bard has offensive power, but will never use it for such unless the future demands it. He is like a guardian of all dimensions. Riot has stated Bard can warp time and space to fix "broken" universes, but chances are it would never get to that point unless a being equally powerful to Bard (only beings coming close being Aurelion and Kindred, being basically the same area of power), but none that would WANT to do such a thing has been revealed yet. Until the Void gets a big lore story and the "Mysterious Laugh" is revealed, I don't think anyone could possibly measure up to Bard in power.
Imagine someone who can warp time and space AND can travel through dimensions. Not only this, but he can also see into the future or knows exactly what needs to happen for things to go the way they are supposed to be, so he can plan ahead almost infinitely. Heck, maybe someone CAN fight and equal Bard lore-wise in the game right now. However, chances are Bard would NEVER let that happen unless the only right way IS Bard's death. Basically, due to how overpowered Bard is lore-wise, the only way Bard can die is if Bard himself allows it or if we have an "anti-Bard" introduced into the lore.
It's like this. You could have a ginormous gun that is basically unbeatable and fires shots that can destroy planets. You plan to use it to fight Bard and/or destroy the universe. However, you DON'T have that gun. Why? Bard. In fact, if you making that gun is impossible to stop without harming other bits of the universe, Bard can go even further back and just stop you from being born in the first place. That's why you can't beat Bard. Immortal, time and space bending god that can see into the future and travel across multiple dimensions. How do you beat that without being an evil version of the same thing?