The lunarch

[Deathstorm]

if I give all me creatures vigilance and attack with {mikaeus the lunarch} can I tap him after def is chosen to pump my creatures?

[InfinitiveDivinity]

if I give all me creatures vigilance and attack with {mikaeus the lunarch} can I tap him after def is chosen to pump my creatures?

Sure, his ability is an activated ability with no written restrictions, you can use it at any time at instant speed unless another spell or ability says otherwise, and of course, as long as he is untapped. All costs must also be payed, obviously.

[Deathstorm]

That's what I thought but I had a buddy tell me otherwise

[InfinitiveDivinity]

That's what I thought but I had a buddy tell me otherwise

Another thing to note is that in order to deal the extra damage, you must activate his ability BEFORE the damage step, I would recommend doing it before passing priority after blockers are declared, otherwise your simply putting counters on them post combat.

DECLARE ATTACKERS

                V

DECLARE BLOCKERS
        (sweet spot)
                V

         DAMAGE