I played as a caster intelligence hero. I had frostbite, brilliance aura ... and formless' ultimate skill. I learned duel and shapeshift. And when I clicked the spell, it's gone, hidden or broken. I couldn't use this skill anymore. I played with 5 skills in s6 mode.
1. Essence Shift
2. Berserker's Blood
3. Atrophy Aura
4. True Form
5. Craggy Exterior
6. Shadow Dance
With this build you can kill any carry hero, just pick agility hero and buy attack speed & armor items and you go!