title says it all
its annoying to fight a heimer solo mid

so yea, how do you deal with those? health regen items?

Early game regen is the best you can do. Late game you have several options.

#1 gank his squishy butt over and over.
#2 Banshee's Veil
#3 Force of Nature (regen + mp resist is nice)
Other MP resist items, and HP / HP regen items.

It has a fairly long cd, so you just need to have enough HP to survive the burst, and then regen/mp to pop you back up to full before it recharges again. The worst thing you can do is let Heimer feed, especially if he goes for a Soulstealer. Heimer + Karthus Ult is truly Imba if played well together.

Well, I play Sivir (as most of you know) so I figured Id add it in.

Sivir is the hard counter to heimer in mid. Just ensure that you get 3 points in spellshield and every missile volley he does will just feed you mana. The grenade you can easily dodge (or spellshield it if you really want to). Just remember to run sideways, not backwards as thats where it is usually targetted.

Ricochet and Boomerang Blade were practically MADE to kill Heimers turrets AND harass him at the same time.

I have yet to face a heimer mid who did not quickly call for a lane change. All it takes is a couple spellshields followed by BB's and you dont see heimer again till late game.

EDIT: Just in case it wasnt implied, this takes mastery of spellshield. It is not for noobs (Sivir isnt in general, despite what her IP cost and difficulty rating say).

merc treads if there's more magic users on enemy team
negatron cloak/banshee's veil is almost always useful
hp items if needed

otherwise switch with someone else or call/ask for ganks

If Heimer isn't going turrets as his primary, he's screwing himself over by denying himself early game farm. Dealing with missile harass is no big thing, especially if you have teleport. You can take as many missiles as you need to, then pill back, get full health, buy a few more health potions, and teleport back all without missing any exp at all.

If he is going turrets as his primary, there are a half dozen simple counters. Veigar, Sivir, Nidalee, Annie, Ashe, Twitch, and a lot of other common solo mids can all annihilate heimer turrets quite efficiently (though twitch usually pops his ult before trying large groups of them).

I am no master player, but when I was going missle heimer mid I had the hardest time with a health regen + lifesteal Katarina who only targeted me with shunpo and bouncing blades, and when i was out of range snapped some hits on creeps to gain health.

I guess it is entirely possible with other heroes, just seems that ones who dont have to spend mana or health on spells work best ... (wait?, theres only one of those =D), just harass him as much as possible, and also what the others said, gank gank gank.

MUNDO is sad that you not remember he not use puny mana bar either.

rush BV, buy a catalyst first because it will give you heals each level up. It will help counter is amazing burst.

Call in early support from either top or bot to gank him. Wait for him to use the missiles and while they are on CD jump him. Either he will ragequit after a few early ganks, switch lanes with someone, or be acouple levels behind and hit like a foam ball.