What level is needed to evolve into Infergon?

Level 48 only makes Helmwhelp eligible. It also has to have Perfect potential, and Good or Elite quality does not qualify. Of the three conditions this is the easy one: a Helmwhelp at 48 with the wrong potential rating still cannot evolve, however long you level it.

Infergon is the final stage of a three-stage line that starts at Jawling, and it is one of two branching evolutions in that line. Evolution is the only way to get the standard Infergon. The one exception is Prismana Infergon, which came as a Season 1 reward: an egg earned through stamps in the Season Journey on reaching season rank 12.

What else you need at level 48

Bring a Blaze Stone, because the evolution uses one. Starting the evolution at the sword sets off a fight with Omega Infergon, and you have to win it. You can't capture Omega Infergon. It is the boss at the end of the Russet Highlands Warrior story quest, a storyline that opens with a helmet quest from Painter Sora. Once that quest is finished, go back to the arena and the fight is available.

Perfect potential is the hard part, so work on it before you reach 48. Perfect stats carry over through evolution, which means the usual approach is to catch 20 to 30 Jawling, keep one with Perfect potential and level that one. The Jawling questline and its spawns are in the Russet Highlands around the Sanctum First Site.

Is it worth the climb

Infergon is a Fire-element DPS Aniimo and a flying dragon with Fly III for traversal. Flaming Claws does most of its damage, and Lava Breath applies eight stacks of the Fire debuff. In a fire team it works as the main damage dealer against bosses, and Ferocious Fang is a recommended held item.