I used to feel the same way as you. However, I now view Mischief and Decay to be the two easiest heads to deal with. Wrath is by far my most feared head.

The thing about Mischief and Decay is, putting aside their unique moves, they are the only two heads that are single-target. The other four all have AoE attacks. That makes them so much less threatening. The only thing you need to worry about is dealing with their moves. They do basically no damage.

For Mischief, the easiest thing to do is run someone with ACC down. My main team doesn't even use particularly high resists - they're all around 250 RES, which is pretty easy to gear for. My secondary team instead uses a resist tank (Elder Skarg or Saito), basically built around the fact that they self-buff and are therefore the target. Mischief just attacks one of them, does nothing, and I continue on my way.

Decay, on the other hand, is even easier to deal with. Just run someone with Provoke. I use Maulie on my main team, and on my second team, I use Sandlashed Survivor, with Ignatius providing a backup provoke in case she loses control. As long as Decay is provoked, he does nothing.

These two are annoying for sure. For mischief I use someone with block buffs, for me it's Uugo, and I almost always have a Geomancer on my team to handle both of these heads. Even if mischief steals some buffs it won't matter if the other heads have the block buffs still. 

Mischief has become the easiest head to deal with now. Hex from Mithrala on one team with Lydia block buffs on it... and the other team runs some resist and accuracy down, plus an Uugo with Block Buffs, and just ignores Mischief completely. 

Wrath I actually like to see, and use his damage to my benefit with Brogni and Geomancer reflects. I purposely pop him to get my swallowed champs back on the reflect :D

Decay I sometimes lose control of here and there with Maulie, depending on the 1 turn versus 2 turn provoke trigger. So he can be a real pain sometimes. On my secondary team I run Toragi and he does fair to middling. -_-

I deal most of my damage b4 the heads go with HP burn. After that im screwed. Just put a HP burner thats faster than the heads and can survive a few turns, and you should be good.

I haven't got Mithrala or Lydia yet so that's not going to work 😀

@kramaswamy.kr   thanks for the resistance tip I'll try that and @Harbby  with Uugo and Geomancer who I have

All those ways of doing it... Are lies. It is officially IMMPOSSIBLE to do Hydra.