I'm looking for the Hydra in the Twilight Forest mod and have failed to find it for the past several hours. Is there any strategy to finding this Hydra?
Hydras only live in Fire Swamps (or rarely normal swamps). Hydra Lairs are giant hills with one side carved out, they're pretty hard to miss if there's one around.
However, the easiest thing to do is to craft yourself a Magic Map, which will show the location of dungeons and bosses. This is a Maze Map Focus surrounded by paper (and a Maze Map Focus is a shapeless recipe of a Raven's Feather, Torchberries, and Glowstone Dust). The Hydra will show up on the map as a Hydra head icon - it's pretty hard to mistake.