Belittle means to make some feel smaller by saying something mean

'Belittle' is to dismiss or make seem unimportant. It can be applied to people or things. Literally it means 'to make small'. 'She felt belittled by his attitude.' 'She belittled his arguments.' 'Onslaught' is a destructive attack. It can be literal, as in the context of war, or more figurative. 'The prime minister faced the daily onslaught of journalists' questions.'

I agree with previous comments. It is worth noting that 'belittle' is a verb (and can be used as an adjective in the past participle form), whereas 'onslaught' is a noun. :)

Belittle means to say that something is smaller or less important than it actually is.

Onslaught means a large quantity of people or things resembling an attack