Just visit every page of your Battle Deck and check bottom of each page after if you get the avie it will show at bottom of each page.I got these:
Congratulations! You found the 'Bootleg Easter Bunny' Hidden Avatar

Congratulations! You found the 'Carvey' Hidden Avatar

Congratulations! You found the 'Invader' Hidden Avatar

Congratulations! You found the 'Holographic Anime Raulf' Hidden Avatar

Congratulations! You found the 'Party Monster' Hidden Avatar

Congratulations! You Congratulations! You found the 'Untamed Raulf' Hidden Avatar
found the 'Shelley' Hidden Avatar