there is no way to farm z tokens
You can actually farm z-tokens - a guardian character can get up to 50 tokens per day, plus the chance of getting tokens during the summer and winter donation drives. (I got ~50k tokens during the summer donation).
As such, the main purpsose of buying z-token packages is for their additional items, such as Dunamis and Thernda pets or stat-boosting spells.
While there are a few exceptional z-token items (such as White Knight Z or the gauntlet miscs that went rare yesterday), the best use of z-tokens would be the Golden Giftboxes. While they do rely heavily on rng, some of the GGB items are far superior to anything else available in the game.