Engineer

How to unlock: The Engineer is unlocked after buying over 40 drones. This is a lifetime achievement, as you don't need to have 40 drones active in a single playthough.

The Engineer relies on proper placement of Mines and Turrets. Use Tri-nade and Thermal Harpoons to hit enemies from safe areas. Always place your mines and turrets before activating the teleporter!

Tactics

 * The Engineer can lay mines on-the-go, making them an excellent tool for "kiting" (attacking enemies while you flee to stay out of range of their attacks).
 * Tri-Nades don't detonate on contact with walls, although they detonate after a very short period of time (or on contact with an enemy). This means you can bound them off of walls to hit enemies behind  you or from odd angles in a vertical shaft.
 * Thermal Harpoon has incredible range and tracking, easily able to hit enemies far offscreen. This saves the Engineer a lot of travel when only a few enemies remain in hard-to-reach areas.
 * You can safely use Thermal Harpoon in a group of enemies by laying two mines, and jumping while activating the skill. This will give you enough time to resume running at a safe distance.
 * Sometimes laying a mine or two near your Turrets will redirect monster aggro.
 * If you're quick, you can jump off a ladder and drop a mine before grabbing on again, allowing for safe damage. The same can be done for your Turrets.