I think it was the second mission after you get your powers back after they were taken away, you have to defend a van with the lizard cure in it. It is being attacked by enemies that are 425 xp each. That is one level every 6 or 7 enemies.
I went from level 21 to 39 in about 15 min. Unfortunately you get very little spiders. I was hoping to get the black suit. Also I did my highest combo ever. A "Godlike" 405