It's meant to be hard so endgame players don't get bored. It will get easier, but probably not anytime soon. Each head has a vulnerability/antidote. The only one still screws with me is the Mischief. But yeah, it's not going to be a walk in the park. 

It is intimidating at first, there's no question. But all of the stuff you described has ways of being dealt with:

• The respawning as a random head is indeed one of the most difficult mechanics to deal with. Typically you handle it in one of two ways - either your team is capable of handling all of the heads with relative ease, or you just restart the run. Obviously it's better to go with the former, but especially early on, you'll likely have more luck just restarting. IIRC there is a 50% chance it respawns as the same head, and a 50% chance it respawns as a different one.
• I'm not sure what you mean by "all buffs are not removable". There are only two that are not removable - the Blight head's Toxic Cloud buff, which can be handled either by Block Buffs before it is used or by HP Burn being active (this doesn't stop the buff, but it does disable its effects), and Wrath's Vengeance buff, which just has to be responded to, typically by putting an ATK down debuff on Wrath.
• Clear debuffs, which is applied by the Decay head, is dealt with by provoking it. When Decay is provoked, it cannot use either of its two moves, and instead will just attack the lowest HP champ on your team.
• Steal buffs, which is done by the Mischief head, is dealt with by having a high-resist champ be the target. Mischief will always target the person who has the most buffs, so you usually deal with this by having one champ with a self-buff move and high resist. It is also recommended that you put a Decrease Accuracy debuff on the Mischief head. You can also handle it by using Block Buffs on the mischief head, though I don't like this approach since Mischief also steals TM, and it can be hard to keep the Block Buffs active as a consequence.
• The head you can't typically hit is also the Mischief head, and if that head eats one of your champs, you can deal with this in several ways. Geomancer's passive lets you deal massive damage to any head that has his HP Burn active, and you can apply that HP Burn on Mischief without fear of re-targeting, since it isn't an attack. You can also put a Hex debuff on Mischief, which disables its ability to randomly re-target your attacks. AoE attacks will still damage Mischief. Randomly targeted attacks will not be re-targeted by Mischief, although they also aren't guaranteed to target him to begin with. Lastly, AoE HP Burn does quite a lot of damage, and can be used effectively to break the devour.

I won't requote all that but say many thanks to krakramaswamyfor that very informative information about Hydra, brilliant stuff and really appreciated when the time is taken to help like that

And thanks to harleQuinn who when I got banned for spamming with my second post of the day sorted it out in minutes