In order to play Karma effectively, you must know when to use a Mantra charge.
Mantra on Inner Flame creates Soulflare, Karma's most powerful and dangerous ability, as it one of the most powerful nukes in game (has both spectacular base damage and AP ratio) and should be used in teamfights. It's also the most important part of Karma's combo into a single target.
However, the heal is not very good unless your health is under 50%, but you can use it and follow up with Inspire to protect Karma while she heals.
It also deals a lot of damage compared to any other DoT in the game.
Using Mantra on Inspire creates Defiance, a potent AoE shield and haste which is useful to help an ally surrounded by enemies, as the shield strength and haste are huge. It can be used for AoE damage but mainly to provide durability and haste to your team. It can also be used to absorb a large portion of an AoE ability that you see incoming, as the shield itself is AoE.
Of her skills, Inner Flame and Soulflare have the lowest mana cost and cooldown, making it easy to reduce Mantra's cooldown faster.
Even so, take into account that a higher ranked ability will of course feel more powerful than a lower ranked one when empowered.
Karma's rework was centered around allowing her to be a mage, which was successful. Karma's main combo is:
Land Inner Flame; if you miss it, think twice before following up.
Wait until Inner Flame CD has dropped to 2 seconds (Really easy with 40% CDR).
Use Focused Resolve and land the snare. This is critical, as missing the snare can ruin the combo.
Mantra--> Soulflare; Soulflare is thrown right into your snared victim, shutting down any possible escape from the secondary zone
Use Inspire any time you see a counter-attack heading your way, to either dodge it with the bonus movement speed, or to shield yourself from damage.
Karma's kit is quite versatile, and you should think carefully about which ability to max, depending on the situation.
As her main source of damage, harassment and burst, Inner Flame should be maxed out if you are taking a solo lane or decide to become a severe poking support.
Despite her rework, Karma is still a support and Inspire is a beefy, low cooldown and mana-affordable shield with a potent haste. If you decide to run Karma as a support, it would be wise to be maxing this ability first.
However, it should be maxed last if playing as a mage, as it provides an impressive 40% movement speed boost at rank one, making it a one point wonder. This gives Karma both increased mobility and saving power.
Providing a good amount of damage and useful CC, Focused Resolve is not to be ignored. However, its short range can make it troublesome to use properly, and maxing this ability can waste a lot of damage. If you are facing a melee opponent like Riven or Jax, who needs to be up your face to deal damage to you, the root can be an excellent disengage. Also, in most scenarios you'll easily be able to trigger the snare.
If having trouble sticking to a target long enough to trigger the root of Focused Resolve, slow your opponents with Inner Flame and/or speed yourself up with Inspire.
Gathering Fire is very useful during laning phase, but becomes especially powerful in teamfights.
In teamfights, where you most likely have a high amount CDR, Mantra's cooldown is drastically lowered by Gathering Fire, as Soulflare can trigger multiple times, and can be spammed more often due to the CDR.
Karma can allow incredibly powerful escapes for her allies as well as herself:
Using the massive haste, as well as shield from Defiance, Karma can swiftly get an ally out of danger.
Karma's early game power is surprisingly high due to her high base damages, and the fact that she can use Mantra at level 1 to empower her abilities. A Focused Resolve into Soulflare combo can easily do up to 50% of your opponent's health early on.
The cooldown of Mantra starts when activated, not when the enhanced spell is cast. Activating Mantra in advance of an imminent fight can theoretically save you up to 8 seconds of cooldown on the ability.
In top lane Karma functions well by kiting and zoning AD tank champions with auto attacks and abilities. Inner Flame taken at level 1 allows her to zone effectively and taking Focused Resolve at level 2 both allows her full damage combo and excellent disengage. Karma can burst easily at low levels, but tends to win top lane through forcing her laner to back due to harass and kiting. This also benefits karma due to the ability to get more use out of her passive.
Karma is a support jungler, utilizing her kit to gank early with high burst and CC, and later allowing her team to engage and disengage well in teamfights as well as laying down considerable burst damage while CC'ing targets. Taking Inner Flame first and Inspire second allows karma to farm decently. At level 3 your whole kit will allow you to gank effectively.
Keep in mind while ganking, holding lane, or being caught in an early duel that auto attacking your target will greatly reduce the cooldown of Mantra, and that Renewal will make you deceptively tanky for an early fight and in some cases allow you to engage and gain health during the fight.
Consider buying an Ardent Censer as it would grant 25% attack speed to all allies you shield, also, remember that using Mantra on Inspire will create Defiance, an AoE shield, if used properly on a team fight you can shield your whole team and grant them the attack speed buff.
Karma's abilities can make her deceptively tanky. However since this tankiness comes from active abilities she is especially vulnerable while stunned or silenced, making Mercury's Treads and Banshee's Veil good options when the other team has a lot of CC.
As explained above; Karma's has quite a powerful burst, usually enough to blow up any ADC (which is usually the job of a mage) but can't bring tanky fighters under 30-45% of their health.
The Deathfire Grasp with the initial nuke and increased damage can help with this problem, and the CDR is extremely useful for her.
Karma's benefits greatly from cooldown reduction, which paired with Gathering Fire can burn the cooldown of Mantra in a couple of seconds in a teamfight and can allows the use of your ultimate up to 3 times if you are really aggressive.
Karma's is quite mana-hungry because of the spammish nature of Inner Flame, due to this the Morellonomicon allows her to sustain her damage and improve its spammy nature, as well as providing a good deal of AP
Another option is Athene's Unholy Grail, however, keep in mind that when you buy yourself a Grail a large deal of gold is poured on the defensive stat. This is quite more expensive than the Morellonomicon and does not provide as much as AP as the Book. The mana regeneration offered by this item is also a bit too high for the likes of Karma's
Inner Flame is her main source of damage all game long, and it happens to apply a slow, which instantly procs the improved burn of Liandry's Torment, not to mention that it gives the magic penetration that she is likely to sacrifice by getting Ionian Boots of Lucidity instead of Sorcerer's Shoes, and the bonus health is quite high, considering she has to get really close to deal damage
Either Athene's Unholy Grail or Morellonomicon is recommended due to the fact that having mana and all your abilities off cooldown will vastly improve Karma's zoning and kiting potential, as well as making her much harder to gank.
Due to the changes to Spirit Stone Karma is much more viable in jungle now. In jungle Karma benefits most from early mana regen items with CDR, such as Athene's Unholy Grail or Morellonomicon. The CDR is necessary for clear time and to improve early dueling potential. AP is also recommended to allow her damage to keep up in mid game.
Gathering Fire lowers her Mantra cooldown when she hits enemy champions with basic attacks and abilities. Deny her from getting free hits on you.
Karma's is a difficult champion to finish if pushed to a corner. Don't turret dive her or overcommit unless you can finish her with one or at the best 2 moves.
Soulflare erupts for bonus damage in the area in which it's been placed. React quickly and escape the circle to avoid taking heavy damage.
This ability is usually combined with Focused Resolve to prevent an escape, so play accordingly.
Focused Resolve is a strong disengage tool. Create distance to avoid being rooted and seek to engage afterwards.
Champions you depend heavily on staying on melee range are not recommended against Karma's as she can use Focused Resolve to cut short any engage and heavily punish it with Soulflare or Renewal
Focused Resolve has a very sharp mana cost compared to her other abilities. Try baiting it if you are champion with dashes or blinks. This tactic will drain her mana and leave her without her main CC tool
Carefully watch for the dragons on her back, if they are rotating Mantra is off cooldown.
Karma's is painfully hard to kill by another lone champion. She will play around her Focused Resolve and escape with Inspire, on the other hand she can be quickly killed by an ambush with several champions.
Karma's arms and eyes will be shining brightly whenever she activates Mantra. Her next ability within 8 seconds will be enhanced.
Karma's is especially dangerous at level 3, since she has her whole kit available at this point.
She can actually get her full damaging combo by level 2, be very conservative until you hit a higher level, keep your distance to prevent Focused Resolve and keep minions between you and her to prevent the massive early nuke from Soulflare if she combos it.