The best thing to do is spend the entire game overkilling monsters by using cheer and focus. Or at least starting in mi'ihen. Eventually you'll be able to overkill the monsters without using cheer. Then it's time to move on.

Even if you stop in bikanel, by the time you're in the calm lands you'd still have enough levels built up to have lulu's level three magic.

If you sent kimarhi on yuna's path via rikku, he would have holy very quickly. Level a little while longer on the way to Zanarkand and yuna will have it too, then let lulu use a white magic sphere to get it as well (since she has the adequate mp for casting it)

You can't go back to bikanel at this point in the game, so no cactuars.

You can't get to the Omega Ruins, so no master couerls for teleport/friend spheres either.

Skip the locks for now, if you go mega-levelling it's probably just as well, you'll come back later.

The important thing is getting as many people as possible to know holy. This may just be Yuna and Lulu, or just Yuna.

Whether or not you use the actual spell, just having it will signify the point of levelling that is sufficient to beat yunalesca easily. Unless you're stupid and don't use EVERYONE in EVERY single battle.

Which you should do.

And steal some purifying salts/candles of life from those monk dudes in zanarkand while you're at it.

For yunalesca, keep at least two people in zombie when she switches to her third form. Zombieproof is a waste of holy waters for zombieward and a waste of purifying salts/candle of life (can't remember. one is zombieproof, the other is no encounters) for zombieproof.

Use holy waters if you really need to.

Remember, the key to levelling is overkill. Use aeons (this isn't ff8 afterall) or cheer and focus.