Everything you need to know guide: https://steamcommunity.com/sharedfiles/filedetails/?id=1285303917
Assault Mode difficulty is set according to 5 or 6 tiers. One vehicle for each mode is probably sufficient, but you want to bring the best you got for the tier. In both modes, you need to kill quickly and efficiently to stay ahead of the waves. For air assaults, speed and firepower are essential. For ground assault, armor is good to have, but penetration power and accuracy are more important.