I know the player you are talking about all too well. Until very recently, he made me EXTREMELY salty. I've only started taking fighting games, namely Injustice seriously, and am proud of the level I have reached (although I still have a long, long way to go). Learning all the terminology like footsies, what wakes up, learning how to properly utilize meter, being able to string combos together, starting to look at frame data etc.
It's been fun, and mostly I love playing against online players for practice. To continually run into a player who LITERALLY only spams guns and swordflip is frustrating as hell. That alone is only annoying, but when he is on the mic constantly taunting, claiming what a good player he is...well yeah, that's frustrating.
Thing is...the guy is just scum, pure and simple. He is also delusional. When he started getting abusive and was obviously delusional, I searched him, and I would encourage you to do the same. Over on Gamefaqs, he has numerous threads trying to boast that he is studying this and that, has a 4.0 GPA, runs a radio show, is a world champion weight lifter etc. He is well and truly delusional, and the fact that he spent 20 minutes filming and talking about his rank on the leaderboards is pretty much proof of that. Not to mention he reported a glitch to the FBI.
Back to my point...I was maining Lex, who can have a hard time against Deathstroke, and I was getting frustrated. After seeing how easily top tier players can defeat him, after seeing just how far I have to go....I realized the problem is purely with me. This guy isn't skilled, and the characer he is using isn't great, just easy to spam with. This community is great, and both of us unlike this fool genuinely want to improve our game. I've already had people offer to give me lessons and practice with them.
This player you speak of has put in 16000 matches, 15500 of which are with Deathstroke, and the next highest amount of wins with a character was 33 with Scorpion. Anyone who plays the game that much and hasn't been able to learn even a single other character or a single combo has issues. Just the other day he tried to claim the tree itneractible on Themyscyra was a combo by itself. As m2dave says in this or maybe another thread, anyone who can't get past his gun needs to find a new hobby.
The kid is delusional, so just ignore him. He really is a scrubby player, and while I can't get past him, I plan on ignoring him and all players like him, going back to offline and training until I can make him ragequit without even trying, with solid gameplay mechanics. My advice, and the advice I am giving myself, is not to lower yourself to a level of even being frustrated at such crappy players. Instead, spend that time and effort on getting better.