Just an idea from the ol’ idea mine.
Been playing a lot of Hearthstone (computer card game) solo adventures. In these games, you fight against a boss who has a special ability — maybe a ranged attack or an area of effect spell — they can use each turn. Some have weapons. Some have armor. What’s cool is what happens when the boss’ state changes — they suffer a certain amount of damage, they lose their weapon, etc. Then they often change abilities or get some new benefit. D&D 4th edition had a similar state: “bloodied,” where some enemies would gain new abilities.
Anyway, create a monster that has a dramatic state change. Maybe an axe-wielding orc boss who becomes enraged if his precious weapon is disarmed and he goes full beast mode, as the kids say, gaining +1 Might. Some recon and clever use of Hunter, Lore Master, etc can glean some insight into a boss and find out their abilities and how to avoid triggering new ones.