Just to start, my highest level was like... 52 while doing this. I had, if I remember right, Whiscash, Dialga, Roserade, Luxray, Honchkrow, and Azelf.
Spiritomb was just there... Embargo's no problem, and it just didn't seem to deal damage.
Gastrodon was each fall to Roserade and Giga Drain
Milo was Giga'd, but Ice Beamed back. I sent in Luxray, got a lucky Spark with a crit.
Roserade was too simple. Hello Honchkrow.
Lucario was unusually easy as on contrary with others. Whiscash and quake was enough.
Only Garchomp gave me problems. 14 levels higher than Whiscash, I had to use Honchkrow as Giga Impact absorber just so I could get 2 Ice Beams effectively.
Azelf I think did nothing at all for me but got killed in one hit. However, if I had to choose one besides Chomp, then most likely the Milotic.