Jagged Horror is useful after all
If you are talking about the achievement for not getting hit by a Bloated Creeper, it is not talking about the fish heads.
http://wiki.guildwars2.com/wiki/Bloated_Creeper
Bloated creepers are creatures (norn looking giants) walking around during the event that explode and cause a ton of damage and conditions. You need to avoid these during the Tequatl event; you also have to kill Tequatl, not just avoid the creepers.