As of March 2023, purchasing talents (previously called cards) is no longer working. The only way to get 60 talents is to grind through 30 champions and get them all to Mastery Level 8. This can be achieved by going into a team deathmatch or onslaught training, they give you a good amount of experience if you win. Having other players in your session also allows you to get a team experience boost, so if you know anybody going for this trophy as well, have them play with you or a party to boost the amount of experience you can get out of one game. If you struggle to have 30 champions unlocked, there are some days where characters are unlocked in a rotation for free, so try to level them up first before you do any default characters or characters you have bought.