Jazz Boss 1:
MCE 200 plus Activate Boss. (I think)
Jazz Boss 2:
Combine the Devan Robot Controller and the Tweedle boss and add an Activate Boss.
Undefeatable Queen:
I forgot which MCE number she is... go to Queen, check the Generator box and check the eventID. That is the MCE number to use.
Rocket Turtle:
Get the Rocket Turtle (from the Bad Guys subfolder) and add an Activate Boss. Put some AreaIDs (with ID 0) around him so he can't fly away off the level, then include a path for him to fly along. Sadly, I haven't been able to get him to continue going in a loop yet, but I'm still trying.

Bjarni:
The Rocket Turtle was never used in the official levels, but is a working boss. Like the Beeboy enemy, which works, but was never included in an official level. Or the Freeze Enemy pickup.