After trying other solutions, I came up with my own idea that worked first try. This requires the $9.99 starter pack, which you probably will be getting if you want to save a lot of hours and have a lot of fun!
The starter pack also gets you VIP status which unlocks Diamonds and gets you XP much faster.
Things you need to buy with diamonds for this:
Ultimate Samson(Around 600 Diamonds?)
Ultimate Fuel(I got mine for just under 5000 Diamonds EDIT: I've seen these much cheaper down to 3500 Diamonds)
Ultimate Bucky(less than 500 Diamonds?)
You get 7500 Diamonds with the Starter Pack, so that's plenty to get all those ultimates, and have more left over to get the rest of the grindy achievements easier.
Now, to do the achievement--go to Solo, Survival, pick any AirMech, make sure to equip Ultimate Samson, Ultimate Fuel, and Ultimate Bucky by choosing the category with your
to choose further. (equip Boost if you want that achievement)
Once you have your gear equipped, select the map Crossroads, and
Your target here is to fill every land bridge with Buckys, and every canyon with Buckys as well in the very narrow points. Keep Building Buckys through the waves up to 140+ scattered between the 8 defense points.
Make sure to start with Buckys on the NW bridge, then NE and SW bridges, finally the SE bridge. I put 3 in each location first, then started building those groups up to ~15 each. Try to build and drop Buckys into the piles so that they break down all the fences too! (EDIT: You can shoot these with your AirMech, can be faster if you have the time)
That lets them crossfire at the incoming enemies and helps a lot! Once I had about 15 in each spot, I filled the canyon chokepoints with a few Buckys scattered on the ridges.
Optional: build Honeymakers on the 4 base build points; and 4-8 Hero units to repair your Bucky armies(try to place them behind the army so they aren't targeted first thing when the enemy shows up!)
Optional Achievements: reach level 10 without buying any abilities and winning(I got bored with all the money and kept upgrading my AirMech every time so this took me a while LOL), reach level 15 in a match, not killing any units with your AirMech, and only Boosting during a match(make sure to equip the Boost before starting the match and buy it during the match!)
The Miser achievement in AirMech Arena worth 138 pointsReach Level 10 in any AirMech, and win without purchasing any Abilities. The Max Level achievement in AirMech Arena worth 75 pointsReach Level 15 in any AirMech in an online or offline game. The Boosted achievement in AirMech Arena worth 164 pointsWin a match, online or offline, using Boosters, all in Air mode.
I found it extremely easy doing it this way, and quite fun watching the Buckys work! Try to drop the creeps(walkers that capture bases) closer to the bases for better income and population cap, and you could consider building some stronger infantry to take the bases and hold them better.