This achievement can be completed quickly out in the Far Far Range. A Tarr is created when a Largo eats a plort that is a different type than the two halves of the Largo (i.e. a pink and rock Largo eating a honey plort). Before you pick up the Tarr, make sure all other hazards in the vicinity are dealt with. Any extra damage taken can ruin the attempt.
You don't need any of the health upgrades for this achievement. Simply pick up the Tarr (Hold
) at 100 health, then shoot it out (Press
) a little after it's gotten you down to 20 health. After that the achievement is yours!
Here's my video for all achievements if you'd prefer. Skip to 2:00 for Once Bitten, Twice... Bitten.