First Onyx medal in GOW 3

I got my first Onyx medal in GOW 3 last night.  I kept seeing the field engineer popping up, so I guess it was counting down for me everytime I upgraded/repaired a device.  I got it the same time I upgraded my sentry gun to level 3.  Too bad we were on wave 20 and the board filled with gunkers.  They destroyed my new sentry.  8(  One thing for sure, in a wide open board like we were on, that gets rough as you can get hit from all directions.  The host quit after the 2nd try, so it killed the flow and I left as well.