Every Mayhem, in all versions, the gameplay is exactly the same: 2v2 against every mob fight in each wave (if you're with randoms, sometimes 3v2 on particular waves depending on how comfortable you are feeling), then go to the boss and bladestack an AoE to wipe out all mobs, then finish off the boss.
Other strategies just aren't as effective because each blade applies damage to every enemy hit by an AoE. Traps only apply a similar amount of damage to only one enemy. Shields, and sometimes Weaknesses, are ditched in favor of just doing more damage. DoTs waste the timer.
I can't suggest a nerf to this strategy because there is no possible way of nerfing it that doesn't render either blades and/or AoEs completely useless. Instead, I want to suggest a much larger variety of enemies to encounter so that bladestack AoE is a legitimate strategy against some, but not all, bosses. These are designed around making other strategies viable. The regular version that we currently see will still exist.
Variant 1: Dual bosses
Instead of enemies appearing at Spiral, there will instead be a boss at both Eye and Star. In waves 1 and 2, they will be alone. In wave 3, they'll each have one minion. Completing an outside battle affects both battles, but takes an extra turn to come into effect. This is to give the people outside some extra time to recover and join the boss battles and benefit from their own outside spell.
Variant 2: The cheating superboss
Always alone in every wave, this boss has more health than normal, as well as some extra stats such as Power Pip chance and a small chance to resist Stuns (higher for Ice; Fire can't resist). And if that wasn't enough, the boss cheats depending on what school it is. For example, you can't Blade or Trap against an Ice boss, but every time you put a DoT on it, it removes a Shield from itself. Storm bosses automatically remove DoTs from themself, but gain a Trap if they hit someone with a Shield. You can't heal at all against a Death boss, but it puts a Weakness on itself every time one of its hanging effects is removed by a card. There should be a positive cheat for each boss to reward certain kinds of play, not just a negative one.
Variant 3: A boss in the mobs
I think that this should only apply to wave 3, or else make a boss that's weaker than the ones we see in wave 1 and use those for waves 1-2. Only one boss can spawn per mob wave, and it always spawns alone. This doesn't really accomplish much, it's just to add some extra variance to mob fights as well.
There should also be a notification at the start of the match telling you what boss(es) will be in each wave and how they'll spawn. Sure, you can always change forms, but knowing ahead of time allows you to plan for them, which will make things a lot more fair for the players. I'm also open to increasing the time limit if any of these options take longer than the regular version.
I would like to add a charm to this post but for some reason it doesn't let me. Yes I agree, it would be awesome to see more variety in Mayhem (BM in general too) would love if we can see more different models too, and as you say, bosses that act differently.