First step is to go to the outfitters, choose the "revolver" option under "characters" and choose Pig Josh.
The easiest way I found to do it was as part of a boost - it took, literally, 5 minutes in a private free roam session.
We had 6 or 7 people in the boost, we all outfitted ourselves as Pig Josh and then one of us would throw dynamite into the rest of us - who had grouped ourselves together.
Instant 6 dynamite kills. Once you run out of dynamite, get yourself killed, respawn and throw more dynamite. Achievement will be unlocked in about 3-5 minutes. Rinse and repeat for everyone else.
JUST AN EXTRA NOTE - as Biohazard and Conchie Joe note below - the 25 players have to be human players, not AI controlled players.