Post your team. Also this is what you get for hyping AG Sandrock.
V2 Assault Buster Gundam, piloted by Usso. (Level 49)
Gundam Sandrock Custom, piloted by Quatre. (Level 46)
Methuss, piloted by Fa. (Level 46) (Repair Hype)
Turn A Gundam, piloted by Loran. (Level 46)
Soleil, commanded by Diana. (Level 45)
Wing Gundam Zero Custom, piloted by Noin. (Level 45)
Tallgeese III, piloted by Wind/Zechs. (Level 45)
Vertigo, piloted by Caris. (Level 45)
MazinKaiser, piloted by Kouji. (Level 45)
Gundam Double X, piloted by Garrod. (Level 44)
R-Gun Powered, piloted by Viletta. (Level 44)
GP03 Dendrobium, piloted by Burning. (Level 42)
Blugar, piloted by Jinguji. (Level 41) (Repair + Replenish Hype)
SUMO, piloted by Harry. (Level 40)
Qubeley Mk-II (R), piloted by Puru2. (Level 39)
Nu Gundam HWS, piloted by Amuro. (Level 37)
Blugar, piloted by Mari. (Level 21) (Repair + Replenish, gains levels mad throughout the stage.)
Highest damage dealer is Loran with Soul and Gundam Hammer for a clean 12,000 damage shot. Next highest are Kouji and Usso, who do around 10,000 damage.
Okay, how can I put this. Keep in mind this is no insult to you, and this isn't to make you feel emo. But -
Put bluntly, your team is poor. There are a bunch of SRWs where you can use basically anyone you want and not get punished by it. @G is one of them... unless you do what you did and use only units you like randomly and utterly fail to care about other synergy and effective units. You can use off non-good people, sure, but revolving your entire team around them is a baaaaaaaaaaaaad idea. Some key PCs you'd do well to use are sorely missing (no Shin Getter? That alone would help your offense massively, even though it's frail. No Walker Gallier? Jiron is an absolutely wonderful pilot, and the Walker Gallier has some pretty excellent weaponry and huge versatility with its two forms. BURNING in the Dendrobium? What the hell, man, you should've slapped Amuro or Quattro into it, it's a mech with excellent durability - both concrete and evade - and great weaponry, it's basically a steal and you waste it with a shoddy pilot. No YFs? Macross people are excellent in @G, using at least one of them is a great idea, and they have deceptively good offense. No Zeta Gundam? The Zeta is arguably the damned best Real in the whole game, and the game hands it over to you early. No Great Mazinger? Didn't the Xabungle arc teach you that the Great Mazinger is a very good mech? It's not top tier, sure, but it'd certainly be an ocean better than some of the people you have, since you have so many bottom-of-the-barrel units saddled along), and you have way too much redundant filler. Okay, let's go for a breakdown, unit per unit.
V2 Assault Gundam - Is very very good, this is not the problem.
Gundam Sandrock Custom - There are games where Quatre is good. This is not exactly one of them. I used Quatre in my @G playthrough, but that was openly due to plain fangirlism, and I dumped -loads- of cash onto the mech. I doubt you loaded as much moolah on Quatre as I did, and even after I did, he was just average. His niche as a Resupply whore also doesn't work very well in @G. Granted, if you need a resupplier, he's probably your unit of choice, since he has actual survivability and doesn't fail horribly at killing grunts, but not all that great regardless.
Methuss - The Methuss really fails. There are good Repair units in @G, but Methuss is not one of them. Far from it, it's one of the worst units in the whole game. Fa is a worthless pilot, and the Methuss can't dodge, deal damage nor take a hit to save its life.
Turn A Gundam - It's good, but you should have a better pilot than Loran in it, honestly (although that may not be an option given your stage). It has a few nice niches, at least. Also, Gundam Hammer is your best damage with it? Where is Moonlight Butterfly?
Soleil - Eh, @G battleship.
Wing Zero Custom - It's a decent mech, although it needs focused babying towards EN and cash. However, slapping Noin onto it instead of Heero? Bad decision, Heero has notably better stats, more suited for the Wing Custom, and Soul. Considering how heavy on low-end units your team is, to boot, it feels like this was a poor overall decision.
Tallgeese - Worse than Wing Zero, pretty much. This isn't a compliment.
Vertigo - Fodderish. Not a good idea to use when you don't have the backup to excuse its usage, and it's a late joiner that needs cash to work. Caris is a good pilot, but you could slap him/her/it on a better mech instead of the vertigo.
Mazinkaiser - Arguably the best unit in the game, and I'd raise an eyebrow and whap you with a newspaper if you weren't using it.
Gundam XX - You didn't get the G-Bits, which completely kills the Gundam XX offensively. It's great for dodging, but a lot of Reals are, and Gundam XX has absolutely no reliable offense without G-Bits, while a bunch of those great dodgy Reals can deal actual damage. Not to mention you're not using Paula for the combined XX, but that wouldn't save the mech's offense outside of grunts anyway (it's more of a defensive junction). You can even use Satellite Cannon late without moon reliance, but that's so unreliable that... eh. The mech is probably a good candidate for the bench in this case.
GP03 Dendrobium - Burning? Quattro is given to you for free and he's a far better pilot. The Dendrobium does not work when you toss a crappy pilot in it.
Blugar - Pure resupply/repair unit in @G that can't deal or take damage in any way - not a good idea. Unit slots in @G are too valuable to use on units that are this frail and inept at slugfesting, and you don't need a whole lot of healing.
SUMO - Man, what is it with the Gundam fodder fixation? Harry isn't even that great a pilot.
Qubeley Mk II - See above. Puru is even a decent pilot, but the Qubeley Mk II is very much filler fodder. If you have better UC Gundam machines, put her into one of those instead. This also applies to most of the good pilots in poor machines you have, like Caris. Actually, don't you have the Neue Ziel? If you do, try that for, say, Caris, or put Amuro in the Zeta Gundam. Some pilot swapping may help a lot.
Nu Gundam HWS - I guess. Amuro is a good pilot, but the HWS is just okay.
R-Gun Powered - She's worthless, sadly. R-Gun Powered never upgrades ever and it's a poor machine in the boiling of eggs.
Blugar - Um, three resuppliers/repairers with paper durability and epic offense? That's impractical and clogs up slots that would be better used on units with actual durability or offense. I'm struggling to see where resupplying would even be useful on your team Wing Zero aside (serious EN guzzler, but he's more or less the only one who does it that badly in your team. The rest of the team either doesn't need the EN or just doesn't use it), and that's a lot of slots deployed for only one PC. Plus, if you focus your cash on the strong durability stats for your mechs (evade for the Reals, defense and HP for the supers), repair units tend to not see much use due to you suffering very little damage. Going for overkill healing doesn't improve your survivability more than plain stat optimization does in @G, possibly hindering it instead, since most of the repair units in the game are very frail. You probably should stick with the White Ark if you want a Repair unit that won't blow up if looked at funny, and you already have Quatre for Resupply (and he's the one with a shred of survivability). More than that may hamper your overall performance instead of helping it.
Mostly, you have a lot of bottom-tier units, and most of them are very frail, and there is a ton of better people you can slap in their stead, assuming they're not hopelessly underlevelled. Hell, even an underlevelled Banjo in Daitarn 3 would probably be a huge quality jump compared to the likes of Viletta and Jinguji.
NINJA EDIT: Also, if worst comes to worst, remember that experience in SRW scales Suikoden-style. So, you could gameover abuse the map for extra cash and levelling/upgrading the units that are behind, since the cash you gained also carries over on a gameover.