What is the best map and team build to earn the Survival Onyx Medal in Gears of War Judgement?

Good tactic, seems to work most of the time when playing with skilled players. (At least till wave 7/8) But I would suggest a team of 1 Medic, 3 Soldiers and 1 Engineer.

Let the Engineer cover the Wretch side (I find that the Gnasher is way better at dealing with Wretches fast) and have the Medic support the Soldiers. The Engineer can drop a Sentinel when someone slips by or in case of a team wipe of the Soldiers. This way the Engineer can also, given the opportunity, try to fix any remaining fortifications.

(The Engineer could also be replaced by a Scout, the Debuff can be quite useful in the bottle-neck. But in this case I would recommend to use the Scout as support and then move the Medic to the Wretch side. The Scout can take care of the stragglers)