The minimum time possible to beat goldrush defending - no points given, no overtime.

Some tricks that I used:

1. I depended on the other engineer (bot) to place a sentry in a proper spawn-camping position. I was restarting runs where the engineer would place the sentry too far back. I believe that these sentry positions work well against the bots due to the way they aim and prioritize standing next to the cart. The position on the tracks is better, because the higher position results in more bullets reaching the other side of the cart, if the enemy gets behind it.
2. That exact sentry position (on the elevated tracks, closest to the ledge, above the third wooden pillar when you count from the far end towards the point) maximizes the area covered inside BLU spawn without exposing the sentry to undue risks. It is still vulnerable to stickies from the side exits, but with enough RED bots alive, that risk is small.
3. When choosing the initial class, wait for 4 bots to spawn before selecting your class. You will be closest to the spawn exit (possibly map dependent).
4. Placing dispenser and teleporter early allows them to get built while you are running around placing sentry and collecting metal. This guaranteed me a lvl3 sentry; full 200 metal on self + some spare metal in the dispenser
5. I would normally put the dispenser between the sentry and the teleporter (further on the same edge), but decided to help the engineer bot in this case.
6. Approaching the time limit, I started moving to a more aggressive position. Shooting from the front guaranteed that no enemy would reach the cart and trigger the cooldown that causes overtime to be applied
7. BLU team always have exactly 1 spy. If you pay attention to their death indication on the top (left of the timer) you can see when they respawn. Being this close to the enemy base, it was easy to time the appearance of the spies and clear them quickly.