It's takes some out of the box thinking to beat merc. I like the challenge though.
this, you just have to figure out what to build when. I heard, since the last update, the rift barrels don't respawn. if that's true it's probably a lot harder now.
As someone that has beat Starhawk multiple times on Mercenary if any one area is giving you grief try changing tactics.
Ballard's Crossing for one is a tough level from start to finish but often players trigger the next wave of enemies too soon.
Let's take just the snipers after the first wave of Scappers. You can build a Watchtower near the original landing zone and snipe the enemy snipers from were there is the white outline to build a garage or outpost.
You have clear view of the other towers and the A.I. won't even fire back till you move closer towards their towers.
So use distance to your advantage. Next before dropping all three rigs, slowly shoot barrels of Rift to build up walls and top them off with auto-turrets where the Razorbacks will enter. This keeps them from ever landing a shot on any of the three rigs you must defend. Also don't forget to place mines in front of your newly created choke point. Place some repair arms against the walls to keep them up for all the waves of Razorbacks.
Once done make sure you have your team mates loaded into Razorbacks and start towards the lifter. You can run over most of the enemies or stay back and snipe them from a safe distance as you move forward.
Tactics like these get you from checkpoint to checkpoint without too much hassle.
Tip #1: Use the rift barrel glitch whenever possible. LBI removed the barrels from respawning between waves in co-op but you can still do this in the single player.
Boring I know but it gives you time to over build and repair what is needed before the next wave of enemies drop, drive and/or fly in.
Tip #2: Keeping your distance whenever possible. Take the high ground or lay back and pick off the enemy out of reach of their weapons. If you don't just rush in they are easy to pick off one by one. Sure not as fun as "all guns blazing" but if you want to trophy up and get through at least once stay out of the line of fire as much as possible. Railgun is your friend. Use it as much as you can.
Tip #3: Learn were the "next event" triggers are and avoid them till you have some defenses around what it is you are protecting. Like White Sands "Ashes" mission. Don't met up with Jonas in the Saloon till you have placed Beam Turrets near the entrances of the town to deal with Razorbacks, Ox-tanks and Hawks. Keep shooting barrels till you feel you have enough or you hit the build limit. Repair ARMS are your friends in missions of defense.
Tip #4: Create choke points whenever possible. Force the enemy into a kill zone and use Outposts to have a few more guns helping you out. This draws the enemy away from you or their target most of the time and helps thin out the Outcast to make it till the end of a wave.
When I again find time I'll post some videos on YouTube of some of the tougher missions and show how to build to make those missions much easier to finish.