To breed legendary miscrits fast you need to have a lot of epic miscrits which can be used to breed exotic miscrits than legendary. The best place to get them is in the premium zones.
Almost all miscrits elements are available in the premium zones.
Here is list of epic miscrits that are exclusively available here using it you can quickly breed exotic miscrits which can then be used to breed legendary miscrits.
Cave of Mists Sledgehog(Earth), Sapphron (Earth) and Microblast (Lightning)
Shack Lower Basement Boo Blazertooth( Fire ) Dark Nessy ( Water) (Exotic) and Rocker Octavio( Wind) (Exotic )
Miscrian Temple Azteko(Earth), Pujin(Water) and Magixio (Fire)
Moon of Miscria Gravitron(Earth) and Mooncrit(Water/Earth)
Ice Tower Aegis(Water) and Duggy(Fire/Lighning)
Jungle Shores Gemix(Earth) and HoneyBolt(Nature/Lighning)
Emerald Isle Amphibunny(Nature) and Quackers(Fire/Earth)
Using them I have bred almost all legendary miscrits.
Hope it helps.
Special thanks to Muthafar Shlool for sharing this tip with me. He has already got almost all legendary miscrits :)
If you have other tips feel free to share them down in the comments.