What do you mean by "who should you sacrifice"? Do you mean who should you use as food? If so, the answer is always "common and uncommon champs, and rare champs you don't need that are also unleveled except to be used as food".

To answer your "need help in arena" question, the answer is "nobody, because you should just farm 1-man defense teams and otherwise ignore this completely".

So, to your only relevant question - for Doom Tower, the goal is to never give them a chance to attack. This means you need some combination of Provoke, Stun, Freeze, and Fear/TrueFear. So:

• Roric (stun)
• Visix (provoke, speed down)
• Apoth (speed up, healing)
• Harima (provoke, damage)
• Scyl (stun, healing, revive)

That should get you through most of DT. Though it'd be better if Scyl were replaced by someone with freeze or fear, I don't see anyone in your picture with those unfortunately.