Isn't this an on-going process? Why can't we use present continuous tense: " We are waiting for their attack"?
Even past tense here makes more sense. 'Menunggu' itself is more suitable as a verb -ing.
The only context that I can think of where it is possible to use 'we wait for their attack' is: - When someone advise the strategy to the others in the group: "(then) we wait for their attack." >> it will be translated: "Kita tunggu serangan mereka" (inclusive 'we').