GA, the first one?
I've played it once in the past 5 years and I 1CC'd it (this past April, with Ax-Battler. Alone)...
Now, it WAS the first videogame I've ever played and I've played it a ton...
It's easier one-player, which can also be said for Final Fight CD (like you mentioned, he other player gets in the way too frequently). Basically, my best advice is to CONSTANTLY be moving and I mean like a fly buzzing around; if you stand around for too long or stay in a straight line LOS with an enemy for more than one second, they will charge at you.
Also, for the second level specifically, you can dispatch the first set of red Longmoans and Heningers (the strongest version of said enemies) without even engaging them; keep close to the left edge (without walking off of course) and let them charge at you, moving up at the last second; two can be dispatched by having them fall off the edge and a third just needs a single jump attack or charge, to get knocked off.
Following that, the next section, collect that dragon ASAP and the one of you that is on the dragon, keep away while the non-rider kills all the enemies.
DO NOT USE ANY MAGIC UP UNTIL THIS POINT.
Once the last battle starts, both of you should Downward Thrust the standing Gruizya/Storchinaya/whatevers (run, jump and attack after jumping; your character will point their weapon down and strike), make quick work of them and then, wait around until the Red Dragon shoots a fireball, THEN use your magic. The level is done.
It's a game I should really do a walkthrough on or something; I know it like the back of my hand :$