A template for 1st ed Pathfinder, to turn a foe into a Boss Monster.

CR: +2
Initiative: If a boss’s total initiative is less than 15, it changes to 15. If that would cause it to go last in the first full round of combat, it’s initiative improves until it goes next-to-last.
HP: Double total HP
Boss Action: At the end of each round, the Boss gains an additional full-round action. It cannot use this to run, charge, or double-move if it has already done any of those things this turn.
Boss Bash: As a full-round action, a Boss can move itself up to its move and damage all creatures adjacent to it at any point as if it had hit them with a melee attack. If the boss had some kind of limitations on its movement they do not affect this action, but are still in place after it takes this action. This movement does not provoke an attack of opportunity. Alternatively, it can expend a spell or spell-like ability to damage these creatures equal to  1d6 per spell level +1d6 per 2 caster levels.
This ability looks and is described differently based on what the boss is, and does damage of a type the boss can normally do.
Boss Options: Anything other than boss bash that the boss can do a limited number of times per day, or per minute, or per round, it can do twice as often.
Boss Resilience: If a failed saving throw would normally cause a boss to be helpless, unconscious, dead, or paralyzed, and the boss still has the boss action ability, the failed saving throw instead just strips the boss of its boss action for 1d4+6 rounds.
Treasure: Give double treasure.

There you go! All the boss’s numbers and abilities are in the range PCs can deal with, but it’s twice as tough and dangerous, and harder to pin down or isolate!

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