He's the first mini boss that appears on the third island on the adventure map on Level 35.
Unlike the regular Gastosquish that's obtainable, the mini boss Gastosquish on the main map has a different special attack that's called Energy Rush. This gives the monster a status effect called "Energy Rush" which gives him a 20% damage boost and also makes his attacks cost no stamina for three turns. The status effect displays the old Stamina Regeneration symbol. It's unknown as to why it has this different special move instead of the regular counterparts special move, Healthy Minerals, however it could be possible that this was Gastosquish's original special move during the early stages of Monster Legends' release when monsters had different moves and stats.
The mini boss also uses a physical damaging move called Attack which deals physical damage and applies daze to the affected monster, further supporting the theory that the moves the boss had were Gastosquish's original moveset during the early stages of the game, as each monster had a physical attack called "Attack".