bloats and husks can hurt their allies with their bile and fire attacks, if the zed hit is close enough they will turn and attack the bloat/husk (at least clots, cyst, slashers, and not raged gorefast) this also is helpful as the husk's flamethrower attack can clear lots of weak nearby zeds. (can also rage SC given enough time)
closing doors on zeds slow them down (mainly raged FP, raged SC, and Hans) because they pause briefly when running to wait for the door to open (it is VERY brief)
un-raged zeds (not Hans) will take a step back to avoid nades that land in front of them (un-raged meaning almost anything not running at you/teammates) lets you make greater distance from a FP that just finished raging and to extend the non-rage portain of a SC if timed properly (they wont try to avoid nades mid-attack obviously)
try not to heal yourself when the trader opens up, as you do not get medic exp for self-healing, rather heal other people on your team or let the medic on the team patch everyone up for some extra exp.
you can sprint backwards (amazing how useful this is when you get the hang of it. ex. bashing a raged SC chasing you that is about to hit you) you can also shoot while sprinting (shooting will stop the sprint but after you stop shooting you will resume sprinting like normal, useful for clearing zeds blocking your way)
that blue cloud(medic nade) heals you (can not stress this enough, played with a couple berserkers that tried to tank FP/SC outside the 3 medic nade cloud and died)
in early parts of a wave, self heal when you have time (even when you are only missing like 5 hp) to reduce the burden on medics (and so their darts dont lock you instead of the guy with like 20 hp left
)
when running from a SC/FP try running to someone running at you (usually a berserker/medic) as this will pass aggro when they block the FP/SC path (and if they are running at you they are probably trying to take aggro off you anyways)
you can outrun raged SC/FP attacks (at least in solo on a medic/berserker) meaning they will swing/jump at you and miss because you ran out of range of the attack.
the first shot you shoot(after not shooting for a bit) usually has 100% accuracy even when sprinting (useful for sprint sniping, ex. sprinting forward, shoot while still moving forward, sprint a bit more, shoot again, repeat. <- useful when you have a good sense of where your gun aims or if *
cringes* you use a cross-hair)
zeds will sidestep to avoid the husk's fireball (even saw a raged FP do this once)