Best user of Infantry Rush?

Probably Marth, due being a support unit.

I don't think there's any best user of it. You can pretty much put it on any more supportive unit who regularly helps units who have high attack stats.

Someone with low attack or a healer, who is on a team of high attack units that want heavy blade. You know peeps like Eliwood and Siegbert want that heavy blade.

I have one too but am foddering her staff

Support skills arent really going to have a "best user". You'd kind of have to look at your overall team comps, but that said, inf rush seems like a very niche skill with limited usefulness

I run it on my Libra who is S supported with my Tailtiu and also runs Drive ATK 3 in his seal, and it helps her charge up her AoE nuke specials faster while opening up her S slot to run an Attack +3 seal (since she doesn't need to run Heavy/Flashing Blade) and giving her extra stat points while standing next to her.

But as others have said, it's best used on units who are there to provide support (e.g Marth or Male Corrin who provide high buffs through their personal weapons can use it the best. )