The level into the mountains starts off in Marion's Bar, which has an infinite supply of bottles, and you can get the achievement for breaking the bottles on your partner's head.
When you start the area, switch control to Marion and start breaking bottles on Indy's head until the achievement pops. You don't have to worry about dodging any enemies, because they don't spawn until you trigger them. Just use B to pick up the bottles from the bar and use X to hit your buddy. Indy doesn't have to die each time, just hit him 20 times for the achievement to unlock.