This is the mission "The Tombs".
Story-related, you can't miss it.
Theo uses holograms to confuse you. At the end of the mission there will be a few of them, the real Theo has a sniper rifle and shoots - the holograms don't shoot, so make sure you watch for bullet trails to spot the real Theo.
BTW, your lvl 30 build from pre TU-8 will probably still be ok for this mission.