In my experience you don't counter Magnus with big guns in this edition. Spam gun drones and, believe it or not, flamer crisis. That's what kills Magnus.
Magnus has great invulnerable save at 4++ rerolling ones, which he can buff to 3++. Fusion blasters are gamble, but a lot of drones start to hurt him, as he's relying on his armor save. Drone controller and 5 markerlights are required though.
Flamers also wound on fives and have godly amounts of hits even in overwatch. Y'vahra might work too now that I think of it.
-Heresy grows from idleness-