This is really simple. Go to Wily Castle in Chapter 3. Use Growth module on what you're grinding(DO NOT BRING ANYONE ELSE) use a move with high power(rolling cutter, danger bomb, etc) it should OHKO Bass and Disco(OHKOing Disco is important due to attack break + Heart Breaker) you should get 4K exp = four Levels FOUR plus growth module increases level up stat gains