Necroing this with a bit of (hopefully) useful information.
Killing bosses and minibosses with charged attacks gives extra drops.
At first I thought it was just the drill, but this likely came from me seeing it the most in the route.
You can tell if it's the killing blow if they also drop medals like any enemy killed by charge attacks.
During testing of the stage 5A boss, i discovered the bros' drops happened even from regular weapon attacks with certain weapons only, but not from attacks from the plane while the drone wasn't attached. The missiles could also "kill steal" you if you were unfortunate. It seems the rock-paper-scissors stuff might apply to all weapons, not just charge shots, though some bosses definitely only drop things from charge shots...
Drop Table:
NOTE: Permanent life increase drops stack. Enjoy your giant life bar lol.
Starting Area:
-Grim drops nothing.
-Upper zone - Titanix drops a powerup /w drill. A powerup carrier also appears if you stall phase 2 a bit.
-Lower zone - Hammer drops a powerup /w drill. Moha-12 drops a powerup /w mace, however it's the last wagon, not the locomotive. The locomotive is the last bar of health, so try to finish off the next-to-last one with the charge attack.
2A
-Krab drops a double extend /w saw. His cronies drop nothing. I also tried to kill them all /w bomb and it never worked.
-Wardisk drops a powerup /w saw.
2B
-Wardisk phase 1 drops nothing.
-Wardisk phase 2 drops a powerup /w saw just like in 2A.
3A
Dominix drops a powerup /w saw, and drops an extend /w mace (wow, player choice).
3B
-Cannon miniboss drops an extend /w any charge attack.
-Phonix drops a permanent life increase /w saw.
3C
Warmax drops an extend /w saw, and drops a powerup /w drill (seeing a theme here.)
4A
Gryphos24 drops nothing. What a sucker.
4B
Kolus drops an extend /w mace. However it's the cannon you have to kill with it, not the boss itself. The cannon pops somewhere mid health bar 3. So try to charge attack it when it has 4 or as soon as it drops to 3.
4C
The Hammer miniboss drops a double extend /w any charge shot.
The Valkiros Cores drop power level-ups /w drill.
Grim drops nothing.
5A
Grim drops nothing.
Drill bro drops nothing /w mace, an extend /w saw and a powerup /w drill.
Taser bro drops an extend /w mace, a powerup /w saw and nothing /w drill.
Bros share damage unlike Krab and his guys from earlier. Killing them "both" with a bomb will drop a powerup, but doesn't do anything special.
5B
The Hammer miniboss drops a double extend /w any charge shot.
The Valkiros Cores drop power level-ups /w drill.
The Valkiros Gunner boss drops a permanent life increase /w any charge shot.
5C
The three sawblade drones drop extends /w any charge shot.
The Nepton Gunner drops a permanent life increase /w any charge shot.
Grim drops nothing.
6A
Red-Dominix drops an extend /w mace and a powerup /w saw.
Mass-production Ironclads ALL DROP POWERUPS /W DRILL. ALL OF THEM.
Core Unit drops a permanent life increase /w drill.
Grim drops nothing.
6B
Blue-Warmax drops a powerup /w drill and an extend /w saw.
Ogre drops nothing.
6C
The Ragnarox Gunner drops a permanent life increase /w any charge shot.
Grim drops nothing.
