Well, do not spend gems on cards for sanctification. Credits are fine, you can always get more credits.

Avoid mysterious chests if at all possible, they cost way too much for what you get from them.

When buying credits from the fair aim 800k+ credits for 10 gems.

In the labyrinth there are these shuffle tiles which give you free items, they are great for getting blue medals but are very rarely worth spending gems on to double the rewards

Also at the end of the labyrinth. Upon winning you get a chance at getting gems or maze tokens from the extra bosses. Personally I would spend the tokens on the plans for the spirit items or on coins/credits.