Level 30 is the hard mid-game wall. Every Evomon stops gaining Evo EXP from combat at this point - the engine simply refuses to apply XP. The only path forward is the Ascension questline, which is gated by four specific objectives.
First: defeat three summon bosses inside the Petal Pond and Lava Crag instances. These are repeatable; the rewards are Evolution Stones and Omni Stones. Second: own five Evomon with S-Talent or higher - SSS is fine, S is the floor. Third: spin any Adventure Suit slot five times to generate the reroll currency. Fourth: submit the matching Evolution Stone for the species you want to ascend.
Once all four are checked, the Level 30 cap shatters and the Evomon's scaling curve reopens. From there it is a straight grind to Level 90 with Breeder Suit and EXP Fruit multipliers stacked. Petal Pond and Lava Crag remain the highest-yield EXP instances throughout the entire 30 to 90 journey.
Players who want to skip the grind can use the Cannon Fodder strategy: capture dozens of low-tier Evomon, evolve them once, and absorb the Player EXP rewarded at the evolution event. The EXP Optimizer tool on this site calculates exactly how many fodder Evomon you need.