I wish I could tell you what the story of this game is, but it really doesn't have one. You might be able to concoct something looking at the item descriptions or the flavor text for the pilots and mechs, but nothing in game will tell you why you're beating the crap out of hordes of faceless goons in a BattleMech. To be fair, you don't really need one.
As others have stated, this is basically a Vampire Survivors clone. Attacks are automatic, so all you really control is the movement and orientation of the mech. As you defeat enemies, you pick up scrap (essentially XP) until you can level up, at which point you can select an upgrade for your mech (a new weapon, a bonus for a weapon you currently use, a boon for your mech, etc.) Waves get harder and harder and will occasionally be punctuated by a boss mech, a CPU controlled mech with random weapons. Survive till the end of the level, then spend any currency collected on improving your attributes in the Garage.
Speaking of the Garage, it should be noted that you can respec your attributes there for no cost, meaning that if you sunk a ton of points into a stat only to find out it doesn't really help you, you've lost nothing. I love how much this game advocates for experimentation. If you fail at a given stage, you can always try a different combination of pilot, mech, and stats from your Garage. No matter what, there's always a way forward.
Not that the game is without fault, though. As you progress, more upgrades will become available. At first, the upgrades will be quite useful. But after a while, you'll find that you only want a handful and all the ones you don't want will cause RNG to work against you. I understand that being resourceful and using whatever upgrades come your way is part of the game, but a stroke of bad luck can kill a level before it begins.
While I wouldn't necessarily call the game hard, it does require some skill and getting used to. At the same time, a lot of your success and getting good weapons and upgrades that complement your play style. This can make something of a double standard; When I win it's because of skill, but when I lose it's because of bad luck. But it means the game never discourages you. As stated before there's always a way forward, a new tactic you can try. Feel free to respec and use different pilots and mechs. And hey, maybe your build would have worked if you got better equipment. Go at it again until you earn that victory.
At $8, it's a steal. I've had a better time with this game than many $60-$70 dollar titles. After around 14 hours I beat it on it's highest difficulty, but there's always Turbo Mode to try, and various self-imposed challenges. I might not play it religiously, but it's a fun little distraction I can come back to. There's no reason for a mecha fan to not have this in their library.
