Many times in the past I have had conversations with people trying to figure out which World of Warcraft raid boss would win in a fight. The game becomes most interesting when you assume that all of them have their numbers scaled to similar degrees, otherwise of course the old bosses get 1 shot by the new ones and that isn't interesting.
There were always a lot of questions. For example, Azgalor has a debuff that instantly kills the target after 1 minute, so pretty clearly either the fight can only last 2 minutes or bosses need to be made immune to instant death attacks. I generally think that instant death attacks aren't allowed, as bosses are immune to most status conditions and I think it would be fair to put instant death on that list.
Which boss wins the Royal Rumble is not immediately clear to me, but it is pretty certain that modern bosses do much better than old ones. Largely this is because of mechanics designed to force tank swaps. Old bosses just sat there and bashed one tank for the entirety of a fight in many cases, or forced tank swaps just by dropping aggro. Both of these are shockingly ineffective against another boss. Modern bosses tend to have stacking debuffs that are extremely dangerous and if two bosses are just standing there beating each other those debuffs would become dominant, far exceeding the damage of mere autoattacks.
A big part of the challenge is figuring out how smart the bosses are. Star Augur Etraeus, for example, summons a Thing That Should Not Be, and that Thing, if it is anywhere near Etraeus, grants both of them 99% damage reduction. Players simply drag the Thing far away but if another boss is just standing there beating on them then this ability is absolutely brutal.
Some bosses also have healing abilities that are similarly impressive. Unless you can kill the Twin Emperors in just a few seconds they will heal each other to full over and over again with little you can do about it. Their other abilities aren't particularly scary though so I expect that other bosses would eventually stack up huge damage over time abilities that would overcome even that incredible healing power.
In both of those cases though the power of the abilities depends on the location of the critters in question. If the Twin Emperors choose to stand next to each other they are a massive problem. If the other boss is allowed to separate them they would be total pushovers.
I think the best way to tackle this is to simply assume that all bosses stand in a pile beating on whatever has the most threat. If some bosses have outrageous positioning based powers, then let those powers do their thing.
Funnily enough this week in Hearthstone there was actually a Tavern Brawl that emulated this sort of question but for Hearthstone bosses instead. There were lots of really neat matchups between wildly different strategies. Chess let you play minions with immense health pools for their cost who attack without the enemy being able to counterattack. The Grim Guzzler let you put a random minion into play for each player, and its deck was full of gigantic minions.
I didn't get enough of a chance to play the Tavern Brawl to be sure of what boss was the best, but I absolutely love the idea. It is one thing to play against these outrageous things as a player, but doing so as a boss is a totally new and intriguing problem.