Before doing a single mission, click the Mail/News icon on the left of your UI and open the Codes menu. Input all active promo codes! This will instantly grant you around 17,000 Crystal Shards and 400+ Trait Rerolls, allowing you to completely bypass the early-game grind.
Go to the Kishke Summoning Altar. Use your starting Crystal Shards on the banner. Your goal is to pull at least one Mythic or strong Legendary unit. Avoid spreading upgrades around Common/Rare units unless absolutely necessary.
Build a team and start the Story Mode Campaign at the "Ninja Leaf" zone. Clear this normal mode using your strongest pulls. Save the gold you earn inside the match to upgrade your highest DPS unit rather than placing 6 un-upgraded weak units.
Need to sleep? Before logging off Roblox, enter the AFK Chamber in the main hub. A full 10-hour AFK session yields roughly 5,000 Crystal Shards passively. The game's economy is balanced around you doing this, so don't miss out.