Crystals can be earned in most of the seasonal festivals, you don't have to buy them or join group instance runs to get crystals. The catch with that is the festivals are only up for a limited amount of time. Some festivals will allow you to earn 1 or more crystals per day because you can rerun quests multiple times per day, if you have enough game time per day. Other festivals limit how many quests you can run per day, limiting how many tokens you can get per day. In the Summer festival earlier this year I was earning a crystal every 2 days. In the Spring festival, I was able to earn 1 crystal per day.

I did try the current festival, briefly. I gave up on it because of the lag. If it wasn't for the lag I probably would have another crystal or two across each of my toons that use imbued LI's.

Scrolls of empowerment are the far more tedious item to acquire, mostly because of how many scrolls it takes to max out an imbued LI. Of course, you need to decide for yourself if getting every legacy on an imbued LI to max tier is worth the effort it takes.

At the very least, I'd like to see the number of crystals and scrolls needed to max an imbued LI capped in some way.