How do I get my pokemon to lv50 in HeartGold?

So I beat the 8th gym leader, I killed off HoHo easily because I don't like him. My team is lv40Nidoqueen, lv43Ampharos, lv40 Pidgeot, Lv37 Dragonair, lv42Slowbro, and lv43Entei. On my first try, I beat all 4 of the elite4 easily, but then I get to Lance and he has lv 48+...my question is how am I suppose to get my team to level up to his lv? Because I read guides and all of them say that my team should be at level 50.... help please?