You know honestly... I don't think there is an easy way to deal with him. After he gets airborn and his buffs go up its just ridiculous. He's pretty much as legendary as you would expect a Daemon Primarch to be. He was EVERYWHERE at the LVO and for good reason.
I'd almost say ignore him and just try to win the ground game because chances are his buddies will be summoning in ridiculous amounts of units which are far easier to deal with.
I know when I played the Infernal Tetrad, I focused on a lone Bloodthirster and ignored everything else. I just tried to keep his summoned Daemons out of my hair and played to the objectives. Sadly I didn't get to finish that game, but he spent so much time summoning I was able to get off quite lightly from the Daemon Princes.
However Magnus is a gunboat like no other and will be raking you with unholy amounts of witchfires. He's really a beast...
Its a shame you can assault airborn targets with jump infantry or other flying creatures because that'd be one way.
But yeah I'd say ignore him... focus on the summoned units. The only time I'd engage any of the FMC's is they fail some of their psychic buffs or somehow you ground them. Then I'd throw everything at them.
Not faced too many FMC lists so thats kind of novice advice... but just some ideas.