Using Starlight's Touch at lower charges makes its healing less Mana efficient, but can greatly increase Taric's sustained damage via Bravado.
Rather than saving Cosmic Radiance for the last moment and risking someone dying during the delay, it can be more beneficial to cast it as soon as you know a teamfight is guaranteed to start.
Playing Against Taric
Taric's ultimate, Cosmic Radiance, has a long delay before its effect. Try to quickly assess whether to disengage a fight or attempt to kill his allies before it goes off.
Taric reduces his spell cooldowns by basic attacking enemies with Bravado. Try to kite him in teamfights and punish him for approaching the minion wave in lane.
Starlight's Touch and Cosmic Radiance are AoE spells for your allies, and Dazzle is linear skillshot with a medium-sized hitbox. Try to position yourself properly so you can stun as many enemies as you can while simultaneously protecting and healing your team.
It might be helpful to play Taric without smart casting to see the trajectories of your abilities from your Bastion-empowered allies.
Most teams are divided in two parts during teamfights - the frontline and backline. If you are in the frontline use Bastion on your ADC, and if you are in the backline use Bastion on your tank/bruiser or your APC if they are melee like Diana or Fizz.
Doing so ensures that your spells, especially Cosmic Radiance, can get their maximum coverage and benefits.
Due to the 100% attack speed bonus from Bravado, AD can provide good damage output if you land Dazzle properly.
Bravado can allow you to have your stun up every 4 sec if you have target next to you along with 40% CDR. Keep in mind that even if you miss the ability, you should autoattack a nearby enemy to reduce its cooldown so you can try again.
Cosmic Radiance has a long delay, and most of the time the enemy team will try to kill your carries before the proc. Use it right after you use Dazzle, this will help your carry defend itself for the 2.5 seconds delay and then kill the enemy during the next 2.5 seconds in which they are immune.
Cosmic Radiance can be very effective for hit-and-run tactics. You can use it to discourage enemies from fighting you as your team destroys their tower and then go back. Remember to let your team know they have to go back if they don't want to teamfight without your ult.
Taric functions best as a tanky protector/support. He is most often played as a support Bot lane, although Top Lane and Jungle are also decent roles. As such, items that grant armor, cooldown reduction, magic resist, health or mana should be prioritised over anything else.
The Relic Shield item line is the best choice for tank supports like Taric as it gives health and CDR.
Iceborn Gauntlet and Frozen Heart are both great options for armor. They both give armor, mana and CDR which are all great stats for Taric. The Spellblade effect on Iceborn also synergises well with Bravado and the attack speed slow on Frozen Heart is great at stopping attack speed reliant champions.
Randuin's Omen can be paired with Frozen Heart to cripple attack speed reliant champions. Its critical strike reduction passive also helps in dealing with champions that rely on critical strikes and its active provides another source of CC in the form of a slow. Active and Passives aside, the high health and armor values that this item provides also increases Taric raw tankiness. However, bear in mind that this item, for the most part, only benefits Taric, making it a somewhat selfish purchase.
Thornmail's massive armor stat, along with its damage reflecting passive, makes it seem very appealing. Much like Randuin's Omen however, it only benefits Taric. Also take into consideration that if playing as a support, it is unlikely that enemies will be focusing you over squshier targets which renders Thornmail's passive less effective.
Dead Man's Plate is useful, offering a decent amount of armor and health along with a passive that gives movement speed and a slow. The passive movement speed helps with roaming and also counteracts Taric's weakness to kiting. The slow adds another source of CC albeit a rather mild one.
If solo laning, Sunfire Cape can cover Taric's lack of waveclear and add another damage source. Its jungle item counterpart, Cinderhulk, does not provide armor although the health and AoE damage are useful nonetheless. This item is not recommended for support Taric as the passive might accidentally steal CS or push the wave unfavourably.
Alternatively, Taric can build a mixture of attack speed, AD, crit and on-hit effects to synergize with Bravado's attack speed boost and damage. Although it can do very high damage, building this way is not recommended as Taric lacks an easy way to stick to targets and a lot of his kit will be less effective.
Dazzle is arguably Taric's most important basic spell. Without it, he will lack damage as he can't get close enough to use Bravado and a source of initiation or peel as it is his only source of CC, thus forcing him to play defensively. There are a few ways to avoid or negate the stun:
High range champions like Caitlyn can kite Taric with ease.
As an almost purely melee champion, slowing Taric can cripple him almost as much as hard CC can.
Don't underestimate Taric's damage in the early game. Alongside his Dazzle, he has 100% attack speed and bonus damage from Bravado after using an ability; which technically means that all his abilities do damage, even if indirectly.
Due to Bastion Taric can also make his allies deceptively tanky through a huge boost in resistances. His heals, shields,invulnerablility and Summoner Spells should also be taken into account when deciding whether or not you or your team can kill Taric or his allies safely/easily.
Taric's Cosmic Radiance does not make its targets immune to CC. Holding him or his allies down can waste the ability's duration and waste its effectiveness.