Karma has been a favorite champion of mine since she was released. Although I did play solo mid Karma back in the day I just wasn't skilled enough to build and play her properly.
Now that I've grown I can truly say that Karma is an AP Carry. Not only is she an AP Carry she is the Anti-AP mid. She counters almost all AP mids with a small selection that can give her a hard time.
What is good about AP Karma?
Karma's passive is extremely powerful. The bonus AP you get from being low on health allows you to itemize yourself differently from other AP carries. You can afford to go for the mana regen, cooldown and defensive items without sacrificing damage. I try to keep myself around 75% hp just for my AP bonus from missing health. Rabadon's Deathcap combined with her passive is outstanding!
Since Karma does not need to rush AP, I start with a Chalice of Harmony. Not only does this give me all the regen I need for the rest of the game, it gives me magic resist to shut down most AP carries in mid.
Again, Karma can focus other stats because of her passive AP bonus. I use defensive masteries for bonus HP and damage reduction. This also lets you pick up the bonus gold on kills/assists. Combining this with her shield gives you a very tanky feel when playing her.
When Karma is fed she will snowball with extreme power. Her damage, shield and heals will easily turn fights in your teams favor. If you are obtaining Lich Bane early you will be able to 3-4 shot most squishies.
Even if Karma is not getting kills or assists she can clear minion waves extremely early. At the end of each game my CS is usually a minimum of 200. 300 is easily obtainable if you have a longer game (40-50 Minutes). Clearing minion waves will keep constant pressure on mid, preventing the enemy mid from ganking other lanes or having much time to back away from the lane.
Since you deal less damage at full HP and more damage at low HP this can really throw off your opponent. After trading shots at each other back and forth we normally get a feel for how much damage our opponent can dish out. Since your damage rises when your low this will throw off your opponent and may underestimate how much damage you can do. Your passive also increases at levels 3, 6, 9, 12 and 15. You can use these upgrades to your advantage to finish off an unsuspecting enemy.
Pros / Cons
Strong AP Carry
Tanky AP Carry
AP Mid Counter
Weak Vs. Cassiopeia
More to come soon...
Runes & Masteries
I take mostly AP runes for the extra damage. These runes combined with your passive will let you hurt pretty hard while you knock out your first few items.
Karma's mantra is her bread and butter. CDR is crutial. Combining CDR per level glyphs, the defense tree's "Enlighment" and the offense tree's "Sorcery" this gives us 20.2% CDR. Picking up Morello's Evil Tomb will cap us at 40% CDR at level 18.
Defense - The CDR and extra tankyness is why I choose the defense tree.
Offense - The CDR and magic pen are the main focus here.
Karma's itemization is my favorite thing about her. Her passive lets her ignore early AP items.
To counter the current meta of AP Carries mid, I start with a Null-Magic Mantle and two health potions. This makes most AP carries limp noodles against Karma. They will easily go out of mana before coming close to killing Karma. Try to predict their attack and use your shield to CS and block enemy spells.
I tend to rush a Chalice of Harmony right away, sometimes backing as soon as I have the gold for it. More MR and the mana regen is outstanding. If you choose to farm longer save enough for boots.
After your Chalice of Harmony and Boots of Speed I always go for Fiendish Codex. It provides mana regen, CDR and AP. Truly a great jack-of-all item for Karma.
Remember - Karma's passive allows her to not rush big AP items. You will still hit very hard at this point.
Morello's Evil Tomb or Deathfire's Grasp:
I leave this choice up to you. I prefer Morello's Evil Tomb for the extra stats. If it's your choice, take Deathfire's Grasp instead. Less stats but a clicky for extra damage.
Finish your boots.
Rabadon's Deathcap > Lich Bane > Void Staff > Sell Chalice > Guadian's Angel.
Pretty straight forward, Rabadon's for the bulk AP and percent based boost. Combined with your passive you'll hit extremely hard. Lich Bane adds a full nuke to your attacks every 2 seconds and void staff for the magic pen. When you can sell the Chalice of Harmony for a Guardian's Angel.
As your health gets low your AP should rise close to or more than 600.
Max Soul Shield first (E) than Heavenly Wave (Q). Pick up W when you feel necessary, usually I find it needed or useful around level 6.
I use teleport to finish my Chalice of Harmony early, usually within the first few minutes of the game. After this I save it to help other lanes or to return to mid after ganking a lane.
*This is really a player preference. Replace to your style as a player*
Flash is flash, even when nerfed... still necessary.
Keep 1 Mantra Stored
It is best practice to keep one Mantra stored at all times, in case of ganks. Use your Mantra before you cap out at two. I saved my self several times from a Soul Shield + Mantra'd Heavenly Wave.
Use Mantra Early
After hitting "R" you have 8 seconds to spend your Mantra. You will start charging a new Mantra as soon as you hit "R". You will have more mantra if you predict your uses instead of hitting "R" than "E" right back to back.
Example: I am about to last hit some minions. I hit "R" THAN auto-attack them down to low health. Than I use Soul Shield to last hit the wave. This means about 4-7 seconds of charging a new Mantra have passed.
Your Mantra needs to be used for your shield 90% of the time. It is your hardest hitting ability and provides a very strong shield. In most team fights you will get 2-4 Mantra uses, save them for Soul Shield as this is the ability that will change most fights and save ally champions.
Your target for your Mantra + Soul Shield is critical in team fights. Your target will either be a teammate charging the enemy squishie or the squishie on your team being charged by the enemy team. The damage is obviously AOE and is best used on a target that will hit multiple enemies.
When being offensive I generally pop a Mantra and charge in to use Soul Sheild. Dealing damage and negating the bulk they will return to you. It is an easy way to bully an opponent out of the lane.
When in the laning phase pay special attention to how close your opponent is to your ally minions. You can Mantra + Soul Shield an ally minion that may be easier to hit an enemy champion.
Although this is one of your worst scaling abilities it is a very short cooldown (6 seconds base, 3.6 seconds at max CDR).
Early game the heal is nothing really worth noting. It does give you partial sustain to keep farming. Late game however it is one of the best heals in the game. When pushing a turret you can easily keep your team full life. Healing up after a team fight can be done very quickly. I use mantra on Heavenly Wave as a worst case scenario. As noted above you want to save your mantra for your Soul Shield.
With such a low cooldown spamming it in team fights provides a good portion of your damage output. You can basically cast two Heavenly Waves per one Soul Shield. When farming I tend to Q first to lower the waves HP and Mantra + Soul Shield to clear the wave.
The direct use of this is obvious - to speed up Ally champions or slow down Enemy champions. The indirect or challenging use is the damage it provides. At max rank it has a 305 base scaling at a 0.7 ratio which is pretty strong.
Laning Phase / 1v1 Scenarios
Generally speaking this ability will be your lowest ranked ability. The damage in early game will be fairly low. It can be hard to land in 1v1 scenarios but try to work with it. Place it on minions (ally or enemy) that are behind enemy champions. This is usually only something to try when you are going for a kill and your other abilities are on cooldown.
Team Fights is where Soul Bond really shines. Place on the enemies in the far back or on your ally champion diving in. It should deal damage, slow your enemies and speed up your allies. Move up and down as needed to try hit more champions.
The range on this spell is fantastic. Use it to pop Banshee Veil's.
Early game you should focus on farming, rushing your Chalice of Harmony is key to keep your mana high. Don't be afraid to take a little bit of damage, it will help both your farm and harass. Use your Soul Shield to CS and harass the enemy champion at the same time.
If your opponent is agressive make sure to Mantra + Soul Shield yourself as close to them as possible. This will let you deal damage and take less. They should end up relatively out of mana with your hp still 50% or higher. If necessary just pop your shield without a Mantra but only in the worst case scenario since over using your shield will make you go out of mana quickly.
Bully them. Push them to their turret to hurt their CS. Push them behind their minions to reduce XP. When your flash is up you can get away from most ganks. Be a bit more defensive if flash is down.
At this point you can easily clear minions waves. Push your lane hard and than help other lanes. Keep pressure on mid, you want this turret down ASAP. The more pressure on mid the less their mid can help out or even back to buy. Sometimes the jungler is even almost required to help hold the lane while they buy.
For the most part in late game follow the guide lines listed in each ability section above. You will have Lich Bane by now or are close to it. You can push down towers just as easy as any AD champ. In team fights pay attention to your targets for your spells and heal if absolutely necessary.
Auto-Attack to CS
It could just be me but I find Karma's auto-attacks very easy to CS with. It helps save a lot of mana.