This can easily be done during the Amusement Park boss (dwarf on stilts).
Play as bud and stand next to the boss. He will deal tons of damage. When you are one hit away from dying, press LT to switch to Terrance (if you lose all your health before switching, it will be a game over).
Now, run around while avoid getting hit, but try to keep Bud close enough to the boss so he gets knocked out. Instead of reviving him, climb on the machines in the background to stay out of danger.
Eventually, the boss will call on some thugs to join in. Jump down and attack them (jump kick attack works best). One of the thugs will drop a beer. If you have a full life bar, drink the beer and the achievement will unlock.
If you are missing any health, defeat the thugs THEN drink the beer. This should give you full health. Now, retreat to safety and wait for another wave of thugs. Continue this strategy until you get a beer to drop while at full health.