Can you use {hidden strings} in this way. You tap {mob boss} to use his abillity play {hidden strings} untap {mob boss} tap him again to use his abillity then use the second part of {hidden strings} to untap him?
I assume you mean {Krenko, Mob Boss}?
You cannot. First off, you are not able to activate abilities in the middle of a spell resolving. Second, {Hidden Strings} specifically states it must be "another" target permanent. You are not able to choose the same target twice.
RESOLVED
But you can tap him, untap him and then tap/untapped something else.