The phrase "enemy assault" means when someone or a group of people who are considered foes or opponents attack forcefully and aggressively. Full definition
Levolution is so deeply entrenched in Battlefield 4's design that smaller scale events can happen such as cutting power to buildings, blocking pathways for enemy assault vehicles and other such actions. (egmr.net)
I notice that being in control of a behemoth like that — as opposed to being airborne or limited to tracks — sacrifices some protection, which means you'll have to keep an eye out for enemy assault players. (thatvideogameblog.com)
Another has you weaving between spots on the map that you need to defend from incoming enemy assaults. (thesixthaxis.com)