When I first got started in Mann vs. Machine, I was always the guy to focus exclusively on survivability. I would max out bullet, blast, crit, and fire resistance before getting any damage upgrades. This worked fine for boot camp, but when I switched to Mann Up mode, it left me with some bad habits: I would constantly stand out in the open, not making use of any cover whatsoever. After maxxing out all resistances, only then would I start focusing on damage upgrades. I realized how bad this was once I noticed I was consistently one of the lowest damaging players on the team, even as demoman or heavy. I can't remember who it was that I watched, but after seeing his demo gameplay on Hamlet Hostility, where he got max reload and damage by the 2nd or 3rd wave, I decided to adopt this new strategy. Sure, I started dying a lot more, but that's only because I had gotten used to standing out in the open. Once I started prioritizing crit canteens and damage upgrades early, my damage output dramatically increased, and therefore I became a better asset to my team. Therefore, I would encourage newer players to try adopting a more aggresive approach to MvM upgrades, and only start throwing points into resistances once you max out your main source of damage. The only exception I would suggest is the crit resistance, otherwise, only put 1 or 2 points into bullet and blast resistance.