How would you set out to design bosses? I'd like to take the JRPG approach, with 5 or so boss types:

1. Half-boss, where players do so much damage but the boss just leaves, wounded and uttering curses.
2. Unbeatable boss, where the boss takes the party out in a couple of turns while taking no damage.
3. Story Boss, encountered throughout the game at specific times.
4. Field boss, wandering about without a real aim. Roughly as strong as a Story Boss, but is ultimately optional
5. Optional Boss, ridiculously powerful and will TPK the players if they're unprepared. Only one or two.

Related: what other kinds of boss fights am I missing?

for a half boss (which i use a lot) just make a character with the level and hp the characters could handle, then give them boosted attack and skill stats and some powers they shouldn't have yet, and when their hp runs out they just run instead of die.