I would say either getting useable pokeballs or the level curb. My whole team was 5 levels lower than Jasmine's Steelix or more. Yet I battled every trainer I could walk or swim to in game, even rematched several of them, used rare candy and fought wild Pokemon for extra exp yet I'm still underlevelled :/
I would probably say the lack of proper pokeballs has been more stressful though. It takes a dozen great balls sometimes to catch one furret on low health. This costs me so much time and money. An ultra ball can't be purchased until the last gym, but long before this you have to go up against Sudowuddo, the red gyarados, potentially the legendary dogs too and random shiny encounters. Yet you have almost nothing to catch Pokemon with! Many pokeballs don't work properly due to glitches. The moon ball doesn't have an increased catch rate on moon stone evolution Pokemon or any others-- its worthless. The love ball does the opposite of what its supposed to do. The fast ball doesn't work on almost any fleeing Pokemon like it was designed to.Common status conditions like paralysis modify your catch rate by 0 rather than 8.