League of Legends

Mages are generally pretty bad into tanks, and that's a significant reason why they tend to be weak in ARAM (unless they have great range or are named Brand or Swain). Most of the time you can have viable options to help like Liandry's, Void Staff, and Rylai's as others have mentioned. But honestly it might be best to try to reroll into something else. Usually ARAM teams have too much magic damage, and that makes it easy to itemize against.

I remember seeing a discussion over a premade ARAM team with hundreds of games and 99% winrate. They would roll for a comp with 3 melee frontliners, 1 ADC and 1 mage/enchanter. Sometimes the rolls don't work out like that, but I would try to pick from rolls to fill in those gaps if you want to win. Or just play someone who seems fun, that's good too.