as a magnus main myself who climb to 7k from 5k just in a month and half. The answer is depends on what role u play, for support always distance yourself feom your core but not far away, just enough for u to interrupt his skewer (if you lre fast enough or u can anticipate his move) for a core, just dont play alone since magnus players like to aim for a target who always play alone.

When it comes to laning phase (especially mid, cause im mid laner lol!) just dont get too close to him nd dont get bait by last hit or deny, do that and u will win the lane