You can just do unlimited free Practice Fights against any of your own Dominos (any rank, and then using similar or lower version of those you mentioned you want to try as attacker).Pretty sure those self-practice defenders still carry your own Mastery (so would be like BG, AW, Arena, and actual Duels, with opponent having Masteries, if that matters).(if you didn’t know, click on any of your champs in your roster, drop-down list will include “Practice” to fight against them. Then choose your attacker, just like Duels).
If you want more info can go to this pagehttps://www.guiamtc.com/duels