What causes the cracked floor in Petra to break

If you see a Lifesaver footnote in the PoTG screen, this is intended to feature situations where one player is able to save another player (or players) from imminent death. For example: If Reinhardt has an enemy pinned, and that enemy's teammate stuns or kills Reinhardt, the teammate would be awarded a large amount of "Lifesaver" score. If Mercy has 5 of her teammates downed, and casts her ultimate, the same situation would apply and Mercy would be awarded a large amount of "Lifesaver" score as well.

From the Overwatch wiki:

The player who has the highest score in any one of these four categories at the end of the match will receive the Play of the Game. If the winning Play of the Game category is Lifesaver, Sharpshooter, or Shutdown then the category name will be called out during the highlight intro. If the winning category is High Score, however, no callout will be provided.

McCree is great at medium to close range but not that good at medium to long range.

I treat McCrees almost like I do Reapers: get out of their close range high-damage zone and you'll be fine.

Genji is not a good counter for McCree*; use any hero that'll allow you to do decent damage from out of flashbang range.
Sniper heroes (Hanzo and Widowmaker) are good at this, Pharah can conveniently drop rockets from above on him, Junkrat can also keep him at bay with his traps. Of course turrets (either Torbjorn's or Bation's) are great to keep any hero away.

Every hero has something that they're great at and it will be hard to defeat them if you go up against their best game. In McCree's case, it is dealing damage at his flashbang range. Do your best to avoid that and you should be fine.

_{* Genji is not a good counter because he usually needs a combination of some of his Shurikens, melee and Swift Strike (occasionally two of these will suffice) to kill another hero fast enough. Genji doesn't do well with long battles against other heroes and this combo will most likely alert McCree to Genji's position and get Genji in flahsbang range before he can kill McCree. This is everything McCree wants and it'll force you into the terrible baiting battle you described.}