Canto isn’t really “fair” in a gamemode like this, put simply. It kills half the aspect of strategy and makes it a hit-and-run fest. How to make it fair? Easy.
Just make canto cost a turn slot if you move from where you attacked. I know this isn’t how canto worked in the mainline titles but for a mode like Summoner Duels, it would be a simple, fair solution. It would make canto users think twice before pulling their BS card.