Pro Tip : Upgrade it so you can drop the portal at your current mining spot and warp back to the dome just seconds before a wave hits.
Users often discuss the balance between "greed" and "safety" in this version of the game:
: Focus on your drill before speed. As you go deeper, the rock hardness increases significantly. If your drill is under-leveled, you will waste entire cycles trying to break a single block.
"Upgrade your shield's reflection capability early. It's the most cost-effective way to deal with ranged flyers while you focus on the ground monsters." Technical Note