The phrase "initial onslaught" means the first powerful and forceful attack or assault that occurs at the beginning of a conflict or situation. Full definition
In the year after the initial onslaught of rejections, I pumped close to $ 250k of personal savings and income into my business. (inc.com)
So we will need to be tight in defence for the initial onslaught I think both ManC and especially BvB will throw at us. (justarsenal.com)
If defenses managed to contract and survive the initial onslaught, then they'd be pulled in so tightly that the fullbacks could overlap into acres of space to receive a pass and either shoot or set up a cutback. (theringer.com)