Heimer usually goes bottom lane. But can do ok top as well.
I'd recommend a tanky AP build with emphasis on CDR. He gets 10% free really early on...eventually 15 to 20% but by then its endgame and who cares?
Ideally, take 9-0-21 or similar with blue CDR runes. You start with 15% CDR ideally. Flash,exhaust is typical (or promote instead of flash for bot lane)
Starting with CATALYST works well. Speed isnt AS important bot as sustain is. This makes you tougher, more mana, and gives great regen.
get Ionic Boots for a fast cooldown reduction, you are at 30% now. 40% as soon as you hit level 6.
Negatron cloak to cover magic resist.
chainmail for armor coverage if needed yet.
giant's belt for toughness.
Then Rylais Scepter for toughening up, adding a slow to your spells.
Deathcap for a big offensive boost.
upgrade negatron to Abyssal Scepter for MPen, more MR, AP.
upgrade chain to thornmail or ideally Zhonyas hourglass for AP, armor, clickey.
Re-order the upgrades as needed.
I focus more on Rocket spam to push and harass the enemy. Rank 3 turret is important so you can drop 2 of them. 1 grenade is enough till later for the stun. Save Ult until baddie attempts to charge you. Then your turrets will target baddie immediately and start slowing him, while you Rocket and grenade pointblank.