Foals are very fragile, but they are pretty good at dodging thanks to their small body and Genbu.
Needless to mention their devastating power, I guess foals are not so bad at no-death challenge (Maybe unicorn is worse). Still pretty intense though :s

About the secret room in the castle, I just accidentally got out of the room and lost Master Ball ToT
Originally I planned to get White Jay (MP drain weapon) from Malphas. It's pretty good weapon for foals, and especially good against Tatzlwurm.


Timestamps
0:01:33 Everfree Forest
0:04:37 Froggy Bottom Bogg
0:07:31 Cloudsdale
0:11:05 Ghastly Gauge
0:16:35 Training Hall
0:17:26 Appaloosan Mines
0:23:01 Manehattan

0:30:12 Enter the Canterlot Castle
0:32:12 Cerberus
0:39:05 Siren Gem
0:51:06 Tatzlwurm
0:55:17 The Creature
0:57:57 King Sombla
1:02:06 Nightmare Moon
1:09:58 Trixie
1:13:53 Dullahan
1:19:08 Chrysalis

1:22:15 Realm of Chaos
1:26:03 Discord