。
He smirked at us.
Literal
Us [object-を] seeing, smirk-[adverbial-と] smiled.
にたり is a 擬態語 depicting a knowing, sinister, or smug smile — different from にこり (a simple friendly smile) or にやり (a broader, more mischievous grin). にたりと笑う captures a subtle, slightly unsettling expression. The adverbial と follows the mimetic (as with ぐっすりと, ぷかぷかと). The verb 笑う by itself is neutral — the mimetic specifies the kind of smile/laugh.