Turret placement can be a deciding factor in a battle. Against most enemies turrets are best when they can support each other, but if the enemy has a lot of area of effect damage your turrets can be destroyed quickly. Placing turrets in brush can lead to a surprise attack in your favor.
The success of CH-2 Electron Storm Grenade is very important to Heimerdinger's survival. The slow and stun both are capable of keeping enemies in place long enough to punish them, but it is also the first line of defense against a surprise attack.
Spreading out Hextech Micro-Rockets leads to more reliable damage and more efficiency against multiple targets, but focusing it creates the biggest impact.
Playing Against Heimerdinger
It's better to destroy all of Heimerdinger's turrets with the help of creeps than to fight them one at a time.
Be wary of Heimerdinger's UPGRADE!!!, as he can use it to find an answer to most trouble he gets into. Once his ultimate is down, go in for the kill!
A H-28G Evolution Turret can act as a ward, though this is not recommended since they cannot stealth, auto acquire enemies thereby revealing them, and offline turrets only give the sight range equivalent to that of a Noxious Trap. Use them as such only if you find yourself repeatedly ganked.
Good H-28G Evolution Turret placement is crucial to their effectiveness. For example, placing one at the back end of your minion wave would make the turrets near useless (as the enemy minions are far away from the turret).
Heimerdinger is capable of stagnating the mid lane and taking a turret without ever becoming aggressive. Coordinate with your jungler and attempt to ward all possible paths to mid so that you can be aware of ganks, then handle laning phase by maxing H-28G Evolution Turret and focusing on maintaining a 3-turret spread that will 'catch' and vaporize every minion wave and cause your minions to shove to the enemy turret. This does 3 very important things:
It shrinks the lane down to less than half of its normal length. As a result, the enemy will have to CS under their turret for almost the entire game and you will have some soft gank protection.
The turret formation (and maxed turret level) makes it too hard for the enemy to simply wipe away the turrets. They will focus spells and damage on taking one down, and you can instantly rebuild and punish them with a burst combo.
The constant lane shoving will inevitably cause your minions to chip away at the enemy mid turret. It is possible to bring an enemy mid turret down without ever forcing your opponent out of lane or using basic attacks against the turret.
When set up before minions appear, H-28G Evolution Turret can be used to disrupt the enemy jungler, forcing an earlier recall, and giving info on where the enemy jungler has been.
Keeping turrets clustered is good against non-AoE based champions. Keeping them away from the minion path cuts down the AoE's effectiveness even more. A tight cluster can block skillshots such as Dark Binding or Mystic Shot, but are susceptible to AoE spells such as Tormented Soil.
Hextech Micro-Rockets is an extremely powerful tool for harassment and zoning, due to its long range and high concentrated damage, functioning as Heimerdinger's main source of damage early game.
Hextech Micro-Rockets will hit the first enemy it makes contact with, so try to catch your opponent in an open area before using it for maximum effectiveness.
CH-2 Electron Storm Grenade stun is difficult to land by itself; throwing it after allied disables have been placed guarantees success. Being closer to the target area is also helpful as it lessens the travel time.
Heimerdinger'sUPGRADE!!! greatly buffs the next skill he casts, each with a preferred usage point:
H-28Q Apex Turret summons a greatly enhanced H-28G Evolution Turret that deals splash damage, charges its beam attack faster, and slows its targets. It's best used as a defensive tool when an enemy directly attacks Heimerdinger, or during a teamfight in which you don't need hard crowd control or burst damage, as it deals high sustained damage that makes fighting near it very dangerous for your enemies.
Hextech Rocket Swarm fires a barrage of several waves of Hextech Micro-Rockets in a line in front of Heimerdinger. It's the strongest and burstier damaging ability that Heimerdinger possesses, although H-28Q Apex Turret deals more sustained damage, and thus it's best used when going for the kill on an opposing champion, or during a teamfight in which a large burst of damage is needed.
Beware that even if the missile doesn't stun, getting hit by the AoE will still slow you a significant amount.
Be careful when Heimerdinger uses UPGRADE!!! as it allows him to use an empowered ability. His Hextech Micro-Rockets has incredible damage potential if all of the rockets manage to hit you.
If Heimerdinger throws a CH-2 Electron Storm Grenade into a bush, he gains vision of it regardless of whether it hits something or not. It isn't recommended to stay in the bush if he spots you in this manner.
Heimerdinger is an incredibly squishy champion if caught out. Take any and all opportunities to attack him if he separates from his turrets or attempts to roam.
Additionally, Heimerdinger is weak in team fights if he has no chance to set up turrets. If a team fight begins and he is without his weaponry, have your team stick on him and melt him as quickly as possible.
If he is allowed to, a Heimerdinger with his turrets set up and UPGRADE!!! ready can be difficult to penetrate, as a fed Heimerdinger who is able to set up turrets is one of the most powerful opponents in the game.
If all your abilities are on cooldown, don't chase him around his turrets, as the turret's damage will quickly add up.
Unlike most mages, Heimerdinger can bring down fighters alone by kiting them around his turrets and landing a good CH-2 Electron Storm Grenade.
Try not to push Heimerdinger to his turret, as his passive allow him to hold it for an extremely long period of time while farming easily.
His turrets will also be hard to take down when near a (real) turret.
Heimerdinger turrets have a low mana cost, so destroying them will not affect his mana much, but they are easier to destroy. Be wary when eliminating them as he may be able to punish you with CH-2 Electron Storm Grenade, Hextech Micro-Rockets, auto attacks, and the turret itself.
Hextech Micro-Rockets will hit the first enemy unit in its path. Keeping a few minions in between you and Heimerdinger will negate most of his harass ability.
The rockets slightly arc onto their point of convergence or can spread out from it depending on where Heimerdinger chooses to fire them.
Veigar can be considered a good counter to Heimerdinger when they meet in a 1v1 lane. Veigar can gain AP from Heimerdinger's turrets by farming with Baleful Strike, both of them have similar mobility, Veigar can land his stun more easily, Dark Matter can one-shot H-28G Evolution Turrets in the mid game, and Veigar, in the early game, isn't a pusher. Also, Primordial Burst can finish off Heimerdinger before he can use his UPGRADE!!! if Veigar can land his combo in time.