With McCree get up somewhere where you can see many characters before using his ulti. You can kill pretty much a whole team with it if you can see them all. You can line it up to kill multiple characters. He's not exactly a character I'm super skilled with myself but killing one character is a waste of his ulti because you can kill a lot more people at once with it. Probably your best bet would have been if you could get up high somewhere where you had a view of a few of them. Also you can see the difficulty rating of the different characters in the hero gallery, choose a character and then click on hero info. It is 1 star for easiest and three stars for hardest. McCree is two stars and Hanzo is three stars. It doesn't really matter - I played quite a bit of Mei when I first started and she is three stars - but it can be helpful to work out the differences between characters when starting out.