No Legedary Res Bosses.

Picture this: The party is heisting the fortress of an evil Warrior Commander. At the end, the commander prevents them from leaving. At CR 10, it will provide a challenging boss fight for your level 8 Party. Then, the wizard casts Polymorph. The commander fails its save, and with no legendary res, is transformed into a sheep or something. The party run and escape.

So, how you would handle this, or any situation where a boss doesn't have Legendary Res? How would you avoid save or suck spells like that? I'd really like some advice. Also: Don't you dare say "just give it legendary res." Did you think I didn't think of that?

The Commander could have a special item or ability that lets it force itself back into its normal form after a turn or two. Unfortunately, save-or-suck is just a fact of how D&D works, and there isn't much you can do about it.

Well, first I’d say a CR 10 is not at all a challenge for a level 8 party. The action economy of [x number of PCs] vs. 1 means the commander doesn’t have a chance. Heck if he rolls low initiative, he may not get a turn at all, control spells notwithstanding. But best case, he rolls high initiave and goes before the whole party, and then maybe, just maybe gets one more turn at the top of the second round before he dies.

So, the commander shouldn’t be a solo (the big clue there is he has no legendary actions). He should have some help. Yes, the commander failed the save, but he has friends who didn’t even deal with the spell. And his allies live in a world with magic, they know how this works, the commander’s friends all focus fire on the wizard to break his concentration. 
Or, even more reliably, again, assuming the allies know there’s magic in the world and how these things work. They attack the sheep, do at least 3 hp of damage, and end the polymorph.

You have a few options:

1) Don't play 5e instead play Pathfinder where save-or-suck doesn't really exist and when it does it is almost impossible for boss monsters to fail the save.

2) Don't use solo boss monsters without legendary resistances, in this instance why would a Commander ever be alone? He should have a dozen men with him that can hit his sheep form to release him back to his normal self.

3) Let the party escape easily. Players love save-or-suck spells because they love cheesing fights like this. So just lean in and let them do it, and live with your boss combat lasting only 1-2 rounds. Even better, have the rest of the adventure be exhausting so the party don't have any 4th level slots left or if they do, they have exhausted everything else and were going to TPK if they didn't polymorph the commander (thus rewarding them for not using that polymorph to cheese the exploration portion of the adventure). 

That guy is +7 to his WIS save, and it's a concentration spell that doesn't kill the target.  I'd allow for that possibility in designing the encounter and applaud my players for NOT being murder-hobos (assuming they don't turn him into literally a fish out of water or something) and let them be heroes.  

Let them use their abilities, spells, magic items and such, they chose them for a reason.  Constantly negating the cool things they can do is very frustrating for the players.  

That Warrior Commander that they didn't kill could become a recurring problem for many sessions, feeding information about party tactics to the bbeg, trying to avenge the bitter humiliation of being "sheeped" whatever.  

Just run with it.

Quote from Xalthu >>

Well, first I’d say a CR 10 is not at all a challenge for a level 8 party. The action economy of [x number of PCs] vs. 1 means the commander doesn’t have a chance. Heck if he rolls low initiative, he may not get a turn at all, control spells notwithstanding. But best case, he rolls high initiave and goes before the whole party, and then maybe, just maybe gets one more turn at the top of the second round before he dies.

So, the commander shouldn’t be a solo (the big clue there is he has no legendary actions). He should have some help. Yes, the commander failed the save, but he has friends who didn’t even deal with the spell. And his allies live in a world with magic, they know how this works, the commander’s friends all focus fire on the wizard to break his concentration. 
Or, even more reliably, again, assuming the allies know there’s magic in the world and how these things work. They attack the sheep, do at least 3 hp of damage, and end the polymorph.

 This was just an example. It could be any high-level boss monster with no legendary res.

Quote from PPSS2022 >>

Quote from Xalthu >>

Well, first I’d say a CR 10 is not at all a challenge for a level 8 party. The action economy of [x number of PCs] vs. 1 means the commander doesn’t have a chance. Heck if he rolls low initiative, he may not get a turn at all, control spells notwithstanding. But best case, he rolls high initiave and goes before the whole party, and then maybe, just maybe gets one more turn at the top of the second round before he dies.

So, the commander shouldn’t be a solo (the big clue there is he has no legendary actions). He should have some help. Yes, the commander failed the save, but he has friends who didn’t even deal with the spell. And his allies live in a world with magic, they know how this works, the commander’s friends all focus fire on the wizard to break his concentration. 
Or, even more reliably, again, assuming the allies know there’s magic in the world and how these things work. They attack the sheep, do at least 3 hp of damage, and end the polymorph.

 This was just an example. It could be any high-level boss monster with no legendary res.

 By leaving the boss alive, the have created an opportunity for the boss to come after the party at a level date in a much more prepared state. Next time, give that commander a half-dozen competent underlings, and a magic item or two to give him protections against the kinds of spells the party used the first time. For example, give him a set of Antimagic Armor that he uses to cast Antimagic Field just before ambushing the party with his cronies. 

The third party book Flee, Mortals! from MCDM has a fascinating way of approaching bosses and leaders by giving them a legendary resistance that costs something, be it HP, speed, limited use abilities, or whatever else, and three legendary actions, each one once a day and only one a round, the gives a unique angle to these kinds of combats and makes them unique. I prefer them over legendary resistance because legendary resistance is no fun. Your players get a cool thing and guess what? You can just say no.

Solo boss fights in 5.5e suffer from two major problems:

1. The encounter building rules do not properly account for the power of numbers (5.0 had rules that were extremely annoying to use, 5.5e just discarded them without replacement). A single CR 10 (5,900 xp) is a much easier fight than 5x CR 4 (5,500 xp).
2. Non-legendary monsters aren't built to be good boss fights (even legendary monsters are only mediocre)

In general, your choices for solution are

1. Don't use solo bosses. In your example, a party of 4 level 8 PCs has a High difficulty of 8,400 xp, so you can just pad your encounter with 3xwarrior veteran (total 8,000 xp) and up to 16xwarrior infantry. This greatly reduces the importance of a single failed save.
2. Build custom monsters for bosses.