You need to (obviously) equip Mechanic Baird (LE) as your character. The pre-order Mechanic Baird character does not count. The Forces of Nature DLC is required. If you are going for the Unstoppable Force achievement (also from Forces of Nature DLC) this will probably come naturally. If you want to get this achievement out of the way, this is a fast way to do it.
This method requires either a second person or a second controller.
Start up a private horde. Thrashball is a good map for private horde due to all of the fortifications it offers, but it really doesn't matter. Set difficulty to insane so that you get more credit towards leveling up your fortifications.
Make sure to ACTIVATE MUTATORS, and TURN FRIENDLY FIRE ON. Enable other hard mutators to get more credit towards leveling up your fortifications.
Upon the match starting, have your second player or second controller give you all of their money. Have them aim at you and press B twice to share money. Each time you share money, you donate $500. Do it twice and donate the $1,000 your second player/second controller started with.
Now you should have $2,000. Establish a base and spend your money on fortifications. Use combos that would spend all $2,000 if possible.
The following are the costs of level 1 fortifications:
Command Center: $1,000
Barriers: Small - $500 Large - $1,000
Use combos that will equal the full $2,000 to speed up the process. Example Turret ($1,500) and a decoy ($500) --- OR--- A command center (1,000) and a sentry (1,000).
Once you have spent your $2,000, have both players equip smoke grenades and tag each other. Since friendly fire is on, both players will be downed, ending the match.
If you manage to spend all $2,000 each time, it will take 25 times to do this.