Just putting this somewhere for my future reference.
Round 2 Skyger (Level 1) I used Firefox (Level 15) fire attack. Got two critical hits in a row.
Round 3 Werewolf (Level 4) I used Turtisle (Level 15) nature attack. One strong hit and one critical hit.
Round 4 Bansheep (Level 7) I used Rampage (Level 14) fire attack. One critical hit.
Round 5 Dream Bat (Level 10) I used Fairy Ferret (Level 15) fire attack, nature attack, and nature attack. Fire weak, nature strong, nature critical hit.
Round 6 Frankenswine (Level 10) I used Firefox (Level 15) fire attack. Critical hit.
Round 7 Rockodile (Level 12) I used Ancient Sloth (Level 15) nature attack. Critical hit.
Round 8 Nightmare (Level 16) I used Rampage (Level 14) fire attack. Critical hit after separate tries.
Round 9 Thunder Mammoth (Level 18) I used Aurora Pegasus (Level 15) nature attack. Critical hit and strong hit. This one took two tries.
Round 10 Elephantom (Level 18) I used Rampage (Level 15) earth attack. Two strong hits and one critical hit.
Round 11 Emerald Dragon (Level 20) I used every creature that had dark, all strong hits, never got a critical hit. Finally "cheated" and used Ultimate attack.
Round 12 Steamtrunk (Level 20) I used Thunderhawk (Level 10) electric attack. Two strongs hits and one critical hit.
- NOTE: some of these rounds I went through various creatures that had similar types before winning with the ones listed. Mainly in the last four rounds.