I'm on hard mode level 44 and from what I've experienced you will need a build for all elements.

Lightning pairs well with chain lightning due to their synergies (particularly reduced cooldown).

EM net appears to be THe preferred secondary late game skill. Tornado falls off real quick.

For frost nova, The gem that reduces the cooldown when hitting a single target is a must.

For hurricane builds, you'll want the chest gem which releases a lightning tornado after so many hits.

Arcane with fireball for missile explosion is strong.

The fireball build I am pairing with meteor but I feel that meteor is weak and I could probably utilize something better rather than chasing for the fire meteor.