Back then:
Demos, flank them using grenades. Hold the stickies for ubers and use the explosion force to push them back
Snipers, counter-snipe the other snipers. Make sure you cover each other's bases.
Medics, heal the demos and heavies when they need it. DO NOT use the ubers unless absolutely necessary.
Scouts, run ahead and report about any hidden sentries. Also, disrupt any potential engineer building.
Now:
STOUT SHAKO: 2 REF.
Medics pocketing heavies and using ubers before the round even starts.
Snipers counter-sniping with the Huntsman.
Demos spamming grenades and stickies in a haphazardly order and blowing themselves up.
Scouts trying to take down a heavy medic combo.