Magmus is classic AoE hero killer in this game so,

Skills:
1.Steam
2.Surge
3.Steam
4.Touch
5.Steam
6.Eruption
7.Steam.
8.Surge.
9.Touch
10.Steam
11.Eruption

Adding points and early maxing Steam ability gives you nice dps and ofc escape if low hp.
Basicly strategy for him is based on 1.Surge->eruption->steam
or if ur vs melee its 1.Steam->surge when he tries to escape->eruption

Item Build

Best item build is
1. Abbysal skull
2.Ghost Marchers
3.Staff of the Master
4. Runed Axe
5.Runed Axe
6.Mock of Brilliance

Its expensive but 2x axe combined with Touch gives you max AoE dmg and Mock as supportive item deals nice dps dmg and increases your base attack
This build is mostly usefull on 1v1 or 2v2 where you can farm fast.
Hope you enjoy

And where is da PK?