Which one is Level 88 exactly???
I think its Walk in the park ???
I think it's the level before "A Walk In The Park". If it's after you "tagged" the alien ship... Then go to the room where the "use turret" tac advise is, go across the room and through the door. Then up (or is it down, I don't remember

) the stairs and out onto the roof.
You should be fine from there.
Good luck!