Is this an instrumental version of a song from something? If so I couldn't figure out what.

Also while I'm at it, what anime is ACE ATTACKER from the same game from?

I'm pretty sure Elzam (the character whose theme "Trombe" is)  is one of Banpresto's original characters, so it's likely not from any anime.

Ah, here we go: http://srwog.bravehost.com/elzam.html WARNING: MAY HAVE SPOILERS BUT SERIOUSLY IT'S SUPER ROBOT WARS SO YOU PROBABLY DON'T CARE

It's an awesome song, isn't it? Sounds like a last battle theme.

I'm pretty sure "Ace Attacker" is also a song unique to the games based around original characters.

EDIT: Also, for those curious what the song sounds like, Salamander Factory put up the GBA version from Original Generation on their website:

http://www.salamander.co.jp/sound/listen.html

Too bad the synth version sounds worse than the straight GBA one

EDIT 2: The GBA one sounds better than the Alpha 2 version

Yup, both are completely original.  Though as a curiosity, there is a vocal arrange version of Ace Attacker on Super Robot Wars alpha Original Score III: Soldier Chapter [FSCA-10131], so maybe you heard that at some point but don't remember it? Sadly there are no versions of Trombe! other than what you hear in-game.

It warms my heart to see people talking about SRW's so very underappreciated music. <3