It’s a problem with large parties. There’s a concept called “action economy” that stops encounter difficulty from being completely linear. A party with more players has more actions, which gives them more versatility, more opportunities to apply different Banes, and more chances for a series of dice explosions to kill the boss in one hit. The boss meanwhile has its focus split 8 ways; while a lone boss could probably beat a lone PC, that same damage divided by 8 is really not very effective.
The solution is as @Great_Moustache says, make the boss a lower level and give it some allies. Personally I favour having one or two higher level lieutenant NPCs and then a mob of minions to keep things varied.
The level of a boss is worth 4 times the level of an equivalent minion for balancing encounters, you are correct. I would use a level 1 boss with plenty of feats and defenses skewing towards the high end, two lieutenants with 6 levels between them and 2 levels worth of minions for a level 12 encounter designed to make the players think but not give too much risk of them failing. That’s just a guideline though, you know your party better than we do.
Remember to make the environment interesting, with plenty of cover and terrain so it doesn’t just turn into chess, and make the enemies act in different ways depending on their skills and behaviours so it doesn’t turn into just standing in the middle rolling dice at each other. Make it memorable for your party, give them a chance to flee but a reason to keep fighting, and do come back and let us know how it goes.