I do not know what you can't an cannot not do but I think the description speaks for it self, just put on that infinite ammo and let it do the work. Also 100% sure QTE's do not count. Not entirely sure about sliding or busting boxes.
As far as I'm aware from the various times I've gotten this medal, you're allowed do everything *except* pressing R1 when it symbolises an attack. For example when you're next to a crate, the R1 prompt is greyed out somewhat and breaking it doesn't count towards melee. But when you're next to an enemy, part of it is red. So if you use standard attacks, counter, stomp, CDG, stealth attack etc. you won't get the medal. But QTE's don't count. Or at least they didn't when I got it. That said, medals are bugged as hell so you could have a perfect playthrough and still not get it!
Actually I forgot to mention, I think that quick shots may count towards melee. I've had several play throughs where I was half-heartedly trying for the medal, and I really can't remember performing any melee attacks but I used quick shot persistently throughout. Haven't had a chance to test it for sure though. Sounds weird but seeing as it requires an R1 and uses up stamina it's not beyond the realms of possibility.