I'm dealing with this right now. My group of 8 players at 7th level are about to fight a CR 16 legendary creature (I combined the stats of the archmage with the fomorian -- it turns out that giving an 18th-level a big bucket of hit points is pretty useful, and giving a giant globe of invunerability likewise).
1. The PCs are attacking the guy's organization to eliminate his most powerful minions before they fight him. They'll also bust some of his enemies out of prison and have already recruited some other allies for help.
2. The PCs are planning to ambush him. Details are still up in the air, but the bad guy doesn't know they're coming, so they can make the fight happen on their terms.
3. They've been tracking down helpful magic items (in particular, they got their hands on a staff that can cast globe of invulnerability, which is stupidly useful against an archmage).
4. This particular villain isn't very tactically-minded. He'll use his spells smartly but I'm not going to be super concerned about having him make the "right" decisions involving positioning or action-denial.
5. IF the bad guy wins, he won't kill them (or at least not most of them), he'll take them prisoner. Then, they have allies who are capable of busting them out of jail. It would be a major setback and change the tone of the campaign, but the PCs are only 7th level, so they have plenty of time to bounce back.
6. IF the bad guy is losing too quickly, some minions can show up. Even though the PCs are eliminating most of his minions and separating him from the others (or trying to), he's got at one minion who is difficult to track down and could plausibly show up unnoticed and unannounced (or plausibly not show up at all, if the fight is too hard).
Overall I expect it to be a tough fight but with 8 PCs a single round of focussed beat-down is going to really put the hurt on most monsters, so I'm more likely to bring in backup than to go easy on the PCs.
1. The PCs are attacking the guy's organization to eliminate his most powerful minions before they fight him. They'll also bust some of his enemies out of prison and have already recruited some other allies for help.
2. The PCs are planning to ambush him. Details are still up in the air, but the bad guy doesn't know they're coming, so they can make the fight happen on their terms.
3. They've been tracking down helpful magic items (in particular, they got their hands on a staff that can cast globe of invulnerability, which is stupidly useful against an archmage).
4. This particular villain isn't very tactically-minded. He'll use his spells smartly but I'm not going to be super concerned about having him make the "right" decisions involving positioning or action-denial.
5. IF the bad guy wins, he won't kill them (or at least not most of them), he'll take them prisoner. Then, they have allies who are capable of busting them out of jail. It would be a major setback and change the tone of the campaign, but the PCs are only 7th level, so they have plenty of time to bounce back.
6. IF the bad guy is losing too quickly, some minions can show up. Even though the PCs are eliminating most of his minions and separating him from the others (or trying to), he's got at one minion who is difficult to track down and could plausibly show up unnoticed and unannounced (or plausibly not show up at all, if the fight is too hard).
Overall I expect it to be a tough fight but with 8 PCs a single round of focussed beat-down is going to really put the hurt on most monsters, so I'm more likely to bring in backup than to go easy on the PCs.