Worms W.M.D
29 Achievements
GOG
Gym Membership
Finish a game with a worm on more health than it started with.
59.0%
How to unlock the Gym Membership achievement in Worms W.M.D - Definitive Guide
It's very likely you will unlock this naturally. Campaign mission 'Tanks for the Memories' is a good place to try this as there are two health crates near the start and you can easily kill every worm from a distance using the tank.
Otherwise you can try this in a local match. Set health crate drop to 100%, pick one up using one of your own worms and just have the other human controller team surrender.