Wait for lotto. While Ascensions mean we now can restore our Pure Prisms a bit, they are a limited resource best saved for hard to farm Gold materials that even in an event store only numbers 10 or 20 drops, then waiting months for another. Painful when you need 40 - 60 of them for a servant.

Skill gems have a long wait, but as long as you have apples and time (or FGA), you can easily farm 100 or more during a lotto event.

Now, if you only needed a handful of gems to finish someone, I'd say go for it, but if you need most of the gems to be purchased, that is a fair chunk of Pure Prisms that can be saved if you don't need him right now.

I only need secret gems and blue gems of caster and 10 more pages. But yeah, I think it's better off if I wait for a lotto event.

Well, could mass farm Blue gems since all the weekly nodes are open and half AP. Not going to finish Gold though, unless you only need a few. 10 pages are easy enough to farm over a few days even without any you might get from events, Summer should be giving you plenty of them.

Pure Prisms are there for your needs. It is always time you save.

I need about 43 gold gems and 21 blue ones, so I can probably farm the blue ones, and wait for the Lottery event to get gold ones.

Not a big deal. While Merlin does benefit from level 6 and 10 skills in longer fights even just as a one turn invuln support that has battery and buster up is fine. Spending pure prisms on gold gems is painful since they're 3 apiece.

Secret gms of caster and pages have a descent drop rate on the caster gate extreme. Better drop rate that other classes secret gems IME.