The Support's Duel: Sona vs. Alistar

Alistar is a strong support, but for all his CC, he comes with a few weaknesses. He has to go all in when attacking the enemy; Headbutt (range of 650) puts him right onto the enemy champion’s range and Pulverize has a relatively small area of effect. Since Alistar’s only damage is melee, you can punish him when he approaches the AD or support by using ranged attacks/abilities. You can use these traits against him by warding the side bushes of the lane to stop him from getting into range for his Headbutt/Pulverize combo and by forcing him to use his Triumphant Roar by poking his health down. Poking is something Sona excels at, as Hymn of Valor (her Q, range of 700) plus Power Chord (her passive) allows for a surprising amount of damage at relatively low risk. If the bushes are warded, then you know where Alistar is and can keep him away from your AD carry with Q and passive poke.

Since their AD carry was Caitlyn, they have the advantage early game. Winning lanes is Caitlyn’s main strength, while Tristana is a complete monster late game (though she does have a really mean level 2 if she takes her Rocket Jump). Ideally, Tristana wants to get to mid/late game where her strengths shine. Playing near your tower and having thorough ward coverage is the ideal way of getting to late game without feeding the Caitlyn.

With Nocturne being their jungle and Alistar their support, the chance of a tower dive is pretty high. Alistar’s ultimate (Unbreakable Will. 50% damage reduction at rank 1) allows him to tank the tower damage really easily, and Nocturne’s ultimate (Paranoia) allows him to jump right to Sona or Tristana. You can have a better chance of seeing Nocturne’s ultimate coming by warding the tri-brush and near Dragon. Those wards cover the areas where his range covers bottom lane. The best way to avoid tower dives is to avoid Caitlyn’s Piltover Peacemakers (Q), and use Sona’s ultimate (Crescendo) defensively under the tower. By using Sona’s ultimate under the tower, you can quickly turn a gank into killing multiple enemies with the help of the tower.

Tristana and Sona win their lane by playing defensive and only engaging when they have the clear advantage from poking the enemy down. Even if they don’t win lane, if they can avoid feeding kills to the Caitlyn and stay relatively close on creeps slain (CS), then they come into mid/late game stronger.

Alistar's heal naturally pushes lanes by healing minions (unless he's very careful). As such, Tristana should be prepared to last hit minions under her tower. Because of her exploding shot, this can be even more difficult but is the number 1 thing that will improve her CS. Sona can help by weakening minions to make it easier for her to kill them (ex: auto attack each caster minion once).

Since your enemies will inevitably be pushing hard against your tower, try to coordinate a gank with the jungler asap. Sona's movespeed bonus and ult can be especially valuable here.

Finally, be prepared for tower diving. Alistar likes to flash under towers and headbutt allies out. Use Tristana's Rocket Jump or Sona's ult to escape this. Tristana's Cannonball can be used to push alistar back into turret range, and an exhaust or Sona stun can kill him even with his ult.

Also, see Rapida's answer for great advice on warding and positioning. Sona should definitely be clearing bush wards and harassing from the bush as much as possible. This will be very difficult, as Alistar's sustain is incredible.