Horde Mode can be a real pain at some point, so try this:

1. Choose the map "Mercy".

2. Go to the church/cathedral looking building and hide behind the table (don't leave this place unless you don't have any other choice).

3. Build your command post there.

4. This place is good because it has a Boomshot spawning spot very close to the table and because you can also narrow down the passage of your enemies.

5. Try to build barriers in all the 3 entrances or just the main one, do not build any other other fortifications (except for the Sentry Turrent and Silverback).

There you have it, a good strategy to help you on most of the waves.