Sikarsis Tactics

Sikarsis was built to be a heal-stick NPC, so is not the most optimised cleric on the block (which suits him and his religion just fine). Here's a quick guide to using him.

Openers

As his main job is to enhance his allies, this is what to look for when combat begins. He has a number of ways to buff all of his allies at once, so this should be the first thing he tries to do on any turn. If some of these buffs are already active, then maybe look to do something else, but certainly do one of these at the start of the combat (in this rough order of preference):

If the party is not hasted (or, in big battles, even if they are) and the combat looks to be hard, cast blessing of fervor. In the following turns, make use of the free metamagic effects (extend, in particular).
Otherwise the standard opener is prayer.
If the enemy is using a particular type of energy attacks, everyone will love communal resist energy. This is a good spell to extend with his metamagic rod. As with all spells with a long duration, this is a good one to cast before combat even begins, if possible.
If facing evil outsiders, summoners or creatures with mental attacks, consider casting magic circle against evil. The bonuses provided by this spell are incidental (most of the party already has deflection and resistance bonuses), so the real point of this spell is the protection against metal attacks and hindering of summoned creatures.

In battles that look like they might be over quickly (or those that look like some extra help is needed), some secondary spells that last a while can be useful:

If out of prayers or in need of some additional buffing, bless stacks with prayer. This is a good spell for secondary encounters because it provides a bit a benefit without burning bigger spells, and can be cast extended, so that it will probably last through a couple more encounters. Spells that last this long are also good canidates
If bless or a magic circle is already running and you have nothing better to do, casting moment of greatness is a nice trick that lets each member of the party pick a moment to double the bonus granted by those spells (as well as other morale effects). Note that his prayer beads allow him to cast this for free once a day, so use that first.

Group healing

Sikarsis can channel positive energy to heal damage on the hole party many times a day. With two uses of this ability, he can activate this as a move action, which allows him to cast a spell, and still provide group healing. If healing is needed, it is almost always worth spending twice the uses to still cast another spell (maybe another heal) in the same turn.

If anyone is in dire straights, take advantage of how good Sikarsis is a healer. Leave cure light to a wand, but otherwise his empowered healing is a more effective choice. Sweet spot is usually to cast the lowest level spell that allows the addition of all of his caster levels (cure serious at the moment, for about 38 points on average). Note that breath of life and heal are not "cure" spells, so can't be spontaneously cast (and don't get empowered for free).

Fixing things

Next, Sikarsis is a problem solver. He has spells, scrolls, wands and other abilities to clear conditions from allies. Unfortunately, most of these only work on one target at a time. Still, if everyone is buffed, or one ally is really screwed, pull some of these out. When trying to solve a particular problem, look for solutions in this order:

Check to see if one of his special ability touches can solve the problem. These don't require spells and have a decent number of uses per day.
If not used yet today, see if his healing bead can manage it.
If the problem can be cleared up by one of his wands, use them. This might seem counterintuitive, since wand charges cost money and memorised spells are free, but any of Sikarsis' spells can be turned into a heal, so they are slightly more valuable. Using lesser restoration from a wand is a typical action here.
Use a spell to solve the problem, the lower level, the better. Stuff like remove paralysis, remove disease, remove fear can be useful here. If necessary, this is where dispel magic or break enchantment come in, too.
Use a scroll to solve the problem.

When using these abilities, timing can sometimes matter. For example, restoration clears all ability damage, so if a target is fighting something that is doing ability damage, it may be better to wait until that enemy is dead before casting it.

See who needs help

If none of this is applicable, take a look to see if someone needs a boost to confront a situational problem. Ideally, you can anticipate this before combat even starts and hand out strategic buffs before you engage the enemy. Some possibilities here:

freedom of movement: This is good to cast on someone getting grappled, entangled or slowed down.
spell immunity: If you fear a particular 4th level or lower spell for some reason (e.g. you know an enemy you are about to fight is really good at casting charm monster or enervation), cast this on the person most likely to be targeted.
spiritual ally: If someone needs a flanker, or you need to hit a flying target, or you need to hit an incorporeal target, this spell is your friend.
death ward: If someone is facing something with energy drain, death effects, or negative energy, this will help a lot. Stack with moment of greatness.

Against the undead

Don't forget gear

While concentrating on his spells, it is easy to forget that Sikarsis' gear provides a number of tricks. His boots of levitation should be able to keep him above the fray. The prayer beads mentioned above provide free spells. His pearls of power all him to recall a 1st and 2nd level spell he has already cast. His cloak allows him to move silently. His amulet lets him buff his AC or saving throws.