Strategy to Beat River in Horde – GOW2
Here is my latest strategy for beating all 50 waves of horde on the River Map on GOW2. This strategy should highlight a proposed strategy that changes between rounds and repeats every 10 waves.
Waves x1 – x5 (like 1-5, 11-15, 21-25…) should have a sniper in each sniper/torque bow spawn location. The fronts of their buildings should be defended by two more players using the lancer as their primary weapon and whatever weapons they want as secondary weapons. I suggest using the default weapons since the ammo automatically reloads after each round. The final player should be using the mortar. While the mortar is respawning, this player should also be helping to defend the two snipers. The person with the mortar on the bridge and the two players defending the buildings are responsible for healing everyone. The snipers should keep busy sniping the incoming horde. At the end of every wave everyone should try to get more ammo. The mortaree should be picking up a fresh mortar, and the two building defenders should be picking up the grenades in the nade spawn buildings. Below is the map highlighting this stategy.

River Strategy Waves x1 - x5
The next phase of this strategy requires the group to start migrating to one of the two sniper buildings during waves x6 – x8 (6-8, 16-18, 26-28, etc). The mortar should be picked up and moved closer to the building being defended at the end of each wave. The two snipers should now be up in the top level of the building sniping down field and across the river. The other two players should be defending the mortar and the house. The snipers should revive each other and the mortaree and the defenders should revive each other when necessary. There should also be a focus on collecting shields. The shields will be a necessity in waves x9 – x0 to keep the bloodmounts at bay. Snipers defend the bridge and front side, defenders defend bridge and front side, mortar shoots at largest group of enemies and fights closest enemy when without the mortar. Same as before, everyone tries to get ammo at the end of each wave and two people try to get the grenades and the mortaree gets a fresh mortar.

River Strategy Waves x6 - x8
The final phase of this strategy applies to waves x9-x0 (9-10, 19-20, 29-30, etc.). During these waves, the entire group sticks to the one building which is being defended. Do not worry about the mortar during these waves unless it is wide open to go get the mortar. You should have a minimum of 4 shields by now. Two in the doorway to the building, one on the ramp going upstairs and one in front of the most vulnerables sniper. Two snipers should be upstairs trying to take out the horde while crossing the bridge and coming from straight ahead of the building. Its more important to take quick shots than accurate shots at this point as the grinders and boomers will take out the snipers very fast. A third person should hide behind the columns on the second floor straight back from the door and should help defend the doorway. The final two players should be defending the doorway. If a shield is kicked down, those players are responsible for picking them up and holding them in the doorway. Everyone should help defend the doorway if this happens. The only thing that can hurt you from the other side of the shiled is a mauler so they should be taken out when they get too close. There should also be an emphasis on taking out the kantus right away because their ink will down you very fast in the higher rounds. Nobody should ever go for ammo, go for the mortar, or leave the building for any reason during these rounds. You will not make it back. Someone will usually go to save you and they will not make it back either. That leaves three people and they will not beat the horde. Stay in the building.

River Strategy Waves x9 - x0
I drew my strategy on the over head map of River which I borrowed from http://www.infektedmedia.com/gears_of_war_2_overhead_multiplayer_maps/gears_of_war_maps.php which has awesome maps.