The full dialogue in the light novel is:
The characters were discussing about some operation.
I'm not sure of why the author would use the particle は here instead of を. From another post on the usage of 何かしら, I have taken it to mean "some kind of", and in this case, would the sentence mean that, "as for some kind of measure, I think he would strike us" , but as this comment has shown, if を is used instead, then it might mean "I think he will use some kind of measures towards us."
Can someone please clarify the usage of particles in this context?
Edit: A further comment from the above mention commenter explained that it might originally be をは but the を has been dropped by the author to make the action itself be the topic instead.