I used to just buy whatever you need to counter the premade... Using the elite shop mostly, but if they focus on their mercs instead of a toy i'd get a tassadar or anything to outfeed them.
also i'd say, never forget about the teammate
If they're not total noobs to sc2 they know how to micro a bit, not being salty towards the teammate helps a lot into making him back you up.
I havent played marine arena in 2 years tho, not sure what has changed, phoenix/goliath probably got nerfed XD
Whats the average lobby waiting time? I might join back if its not too bad, remember Nomoreluck maybe?