MERCS

At the options screen, highlight Original Mode, then hold A, B, C, and then press Start.

In Original Mode, have a Merc that's low on life pick up a Gold First Aid Kit. The moment their life gauge fills up to full, press Start and quickly select another Merc that's low on life. Press Start to unpause the game, and the other Merc you selected should start to recover life as well. As soon as that Merc's life gauge is full, press Start and quickly switch to another Merc that needs life. Repeat until everyone's life gauges are full.

If you beat Mercs on original mode (not the arcade version) you will see in the end that they will show you where all the hidden areas are in game for each level on original mode.

At the Sega logo, on controller 2 hold d-pad left + down, A, B, C and keep holding until the next screen. The round select will appear when starting a new game.