ukmp much every scout who got really good really fast and went invite in a couple seasons played ungodly amounts of dm/mge every day on top of pugging/scrimming
its just about practice like most anything in life
the only scouts i can think of
dante
sonmychest
and they both had top tier competitive experience in other games
kiridon't play DM/MGE for any longer than an hour or so at a stretch [depending on personal tolerance—some people only ever do 15-20 minutes, some people can play all day; know your limit], you'll start to get tired of it and do worse and become frustrated and then do even worse and etc.
a lot of deathmatch skill comes down to natural talent. if you don't have that, you're going to have to practice a whole lot, and you still probably won't be as good. don't worry about trying to win every 1v1; there's more to tf2 than that.
if you have no prior FPS experience aside from tf2, then dm/mge is ALL YOU SHOULD be playing, if you want to improve your aim.
you only start to do worse once you think you have no chance of winning, set small goals for yourself. set these goals so you will improve a certain aspect of your game.