Mechanic (Civilian) Guide

Role
The Mechanic acts as the team's primary offensive hero, the reason being that the mechanic is the fastest driller from mid-game onwards and the mechanic can build tanks. The tanks are especially important as without them, killing the Overmind becomes extremely difficult. While the mechanic does not have good static defenses and has low supply, their machines are incredibly powerful. The downside is that the machines are susceptible to being kited, weak to symbiote traps, and are expensive to upgrade. Still, the mechanic is essential for any chance for the miners to win.

Skill Points and Items
As mechanic, the skill order is:

9 Engineering -> 3 Offense -> 6 Defense -> 10 Offense

The 9 Engineering is to upgrade your mechanic's drilling machines to level 3. The 3 Offense is you can mount turrets to your drills, so you can constantly drill without worrying about spawn. The 6 Defense is so you can create tanks once you get terrazine. The 10 Offense is to upgrade the tank cannon and Auto Turret that will be mounted onto it.

The ideal setup for items would be at least 3 tanks holding: Level 3 Auto Turret, Level 3 Minelayers, and Level 3 Cannon upgrade.

The defensive items aren't actually necessary as when it comes to tanks, the best offense is the best defense.

The mechanic himself should be holding a Level 3 Geoskanker if the zerg is spamming creep tumors and 100x Light Flares to gain vision for his tanks. Other than that, the mechanic doesn't actually need anything more.

Strategy
Early game, create a mining drill. Keep drilling until you have at least 600/600. Build a command center as it will provide you with 2 supply and the ability to train pilots. Train 2 pilots and build 2 more mining drills when you reach enough funds. Don't worry too much about the command center dying once you get the pilots. With 3 drills you should be drilling extremely fast. You can even get 1 more pilot and drill if you find a terrazine geyser. If you lose track of which mining drill has your mechanic hero, then simply press F3.

Next, make a Service Station (Factory) and buy the drill upgrade. Upgrade the drills for each mining machine until it reaches level 3. Keep on drilling until you reach the Overmind. By that point, you should have enough skill points and plenty of funds to build tanks, assuming you still have your terrazine geyser.

Break the rocks and kill the spine crawlers near the overmind with tanks, as the spine crawlers and symbiote traps are too close to destroy them with mining drills. Now position your tanks close enough to hit the Overmind without being in the range of the Overmind's Ziggers. Use Flares to gain vision for you tanks to hit the Overmind. If at any point the zerg manages to destroy all your terrazine, then simply make a service station for every 2 tanks you have. It should be able to balance out any energy loss of the tanks.

Lastly, it is crucial you pull back siege tanks from the Overmind when an earthquake is about to happen. A bunch of zerg will spawn on the Overmind during an Earthquake and without hefty protection, you tanks will die.

Mining Drill
You only real way of getting income. At least 3 early on and upgrade the drill so you can get that mad cash needed for tanks.

Hellion
Don't bother getting this. The problem with them are that they friendly fire your allies, are expensive, and require constant micro to mine with. You should be relying on auto turrets or your teammates for defense anyways.

Tank
The key thing that makes you useful and necessary for any team. You can mine with it and kill any zerg unit or building easily, provided you have the vision. You need 6 Defense and an enriched fuel station to create one. If all your enriched fuel stations die, then you will lose energy on your tanks at 4x the normal rate.

Also while in siege mode, tanks cannot be a vendor for service station. The items you buy from service station will just drop on the ground, even if tank is right next to it.

Shield Generator
This should only be built when assaulting the overmind or in a big base. They're a very hard counter to the Ultralisk as it will deny their ability to Ultracocoon

Planetary Fortress
While it does a ton of damage, the loss of income by putting a pilot inside a planetary instead of inside a mining drill makes it not worth.