Crafting appropriate combat encounters for large groups

[Gerion of Mercadia]

Well, if you are dealing with a larger party; just use proportionally more monsters. Mass combats actually have higher CR because of the risk of somebody getting "ganged up" on. It forces a change of tactics on the part of the PC's and the attackers.

As far as the BBEG - once the party leaves the "fragile" stage and is at least 4th level, they have a greater chance of surviving for a round or two if they get "bushwacked" by a REALLY BBEG.