This build is meant to be played with high confidence, the rune pages the 2 mana runes thingy with doran ring is enough for heimer, and his passive is just enough to dodge spells so you don't need boots, it costs too much anyway.

In short, play really well with your turrets, and reserve your rockets and grenade until it's worth it and you REALLY know you won't waste the mana.

To manage his mana with this build you hsould do the following with the spells:

turrets - this is what makes you the best dps of the game, its got a low mana cost and insane damage if you manage it correctly, try to use 2 turrets for pushing/harassing and 1 defensive turret behind you to make up for your boots that you will never buy

rockets - use it to put pressure on the ennemy if he gets too cocky, try to match your mana with the enemy, if you have the same amount, you can go for it and poke him with rockets, the best thing i do with them is cancelling their backs, never let them go back to base and make them lose time !

grenade - use it defensively, if the ennemy looks like he mana hit you, hit them back with the grenade, even if you get touched you have more DPS with this build, in trades i try not to but it is possible to facetank many champs in this game

ultimate - turret 80% of the time, grenade 19%, rockets 1%, why ?
-turret: usefull in all situation, you see a gank coming ? put your ultimate turret, if they try, they die. Also good utility for pushing and catching the ennemy off guard undertower
-grenade: use it to engage in a teamfight and when you can at least stun 2 ennemy
-rocket: meh its for KDA players, not great utility , it's just for 1 kill most of the time, i prefer to finish them with vanilla rockets or let them go most of the time

You play should with the goal to perma push the wave and leave little to no room for your ennemy to breathe, this will attract the ennemy jungler and you should have the attention of the team near 100% of the time because of how dangerous you will be

With this build you will have so much damage that the ennemies will always think twice before coming to you so your turrets will just do their stuff, push and poke and when it's time go all out

a good tech is, when you see a wave cannon, take advanteage of it by waiting for the cannon to be focused by the tower, then put all of your turrets available and your ULT turret

this can used when you want to kill an ennemy or if you just want to push, but make

here i missed my grenade off-screen, it's embarassing so i don't show it but this is an example
Play Video