No Pokémon may enter a gym fight at a level higher than the leader’s ace.

Don't overlook the boss fight with . While not a Gym Leader, he is widely considered the "true" mid-game boss. His ace, Weavile, is Level 48 (or Level 47 depending on the version/encounter). Many players set a "soft cap" at Level 47-48 for this encounter to avoid being over-leveled for Volkner immediately afterward. Expert Tips for Level Management

Pokemon Renegade Platinum Gym Teams and LvL Caps. : r/nuzlocke

In the realm of Nuzlocke challenges, few mechanics are as divisive—or as transformative—as Level Caps. While Pokémon Platinum is widely regarded as one of the best games in the franchise for a Nuzlocke run due to its difficulty curve and variety, the implementation of level caps fundamentally alters the DNA of the Sinnoh experience. It shifts the game from a test of patience and grinding to a test of strategy and resource management.