Just make colosseum fight deck with compulsory category slots = one deck should have one common/rare/epic/legendary and Mythic girl
(if you do not have one, can leave it blank; Mythic and legendary slots being interchangeable so that you can have max 2 Mythic per deck or 2 legendary if you do not have Mythics),
Just make coloseum fight distinct from Tower PVP and have their unique fight experience - so that this event becomes more meaningful. This will also LIMIT players getting too many rewards as the total number of fights will then be kind of fixed - which i think is your team's main concern.