Fox can hold up to 9 Bombs, and I believe he was using Landmaster in that matchup. Keep in mind that the lasers have a pretty high rate of fire.
Then brick break should do the trick just fine, it breaks barriers and both landmaster & arwing have shielding barriers in place. Brick break would make it vulnerable to further assaults, the barrier is basically the only thing keeping it intact (kinda like samus).
Light screen would halve the damage done by lasers, sub takes smart bomb damage, brick break to break his barrier and deal some nasty damage (no barriers = gg) and recover to recover damage taken. Brightpowder to boost evasion 10%/light clay to make screen last longer/leftovers to recover 1/16th of hp every turn. With a levitating opponent, fox is bound to miss smart bomb or two and landmaster is not too fast or agile, especially not when turning around. Mewtwo doesn't have this disadvantage, he could just go behind fox and deal damage from there and dance around the landmaster. Pressure would also waste his smart bombs from 9 to 4 (rounds down), one way or another.
Either that or cm/psychic/recover/sub. @Metronome/wise glasses/lefties/petaya/apicot berry. Psychic's 10% chance of lowering special defence is a nice bonus. Calm mind helps mewtwo to tank hits, while sub and recover are self-explanatory. Metronome item gives 10% boost to every consecutive use of an attack, like second attack is 10% boost, third is 20% and so on. Wise glasses boost special attack by 10%, lefties is self-explanatory, petaya boosts speciat attack by +1 when in a pinch (1/3hp left) and apicot berry boosts special defence when in a pinch.
Hm, disable to disable Fox's lasers/smart bombs for a while? =D
I haven't played nor seen assault though, so if landmaster differs too much from star fox 64 in terms or being very good at turning around or shooting behind, please let me know.