I chose the mechanical man because...
1>Complete immunity against all elements (QM's lightning spam which won him the first arena competition, phoenix pyre explosions, cold auras, poisons, etc);
2>Mindless protecting him from enslave & cowardice both of which were very lethal in the first competition. Also protects against soulslay, charm and others.
Recall the battle with Potoman's Void Lord, particularly the point with the Void Lord standing right in front of my SC. If I had chosen a Master Lich or Crone I suspect my SC would have either prematurely fled (like Rytek's SC did in the first competition against Cor's Unfettered) or possibly have been killed by an enemy Phoenix Pyre.
3>Not as crucial, but lifeless protected him from life drain, soul vortex, etc. A mechanical man instead of a master lich means I give up 2Death and 1 misc slot for all these immunities I mentioned above... I saw that as a good deal. Your mileage may vary.
So why not the Siege Golem or Poison Golem or another?
1st, all other options meant I lost immunities and/or slots in exchange for more hitpoints. There were too many ways to kill contestants in 1 round *regardless of hitpoints*, as we've seen. Therefore, hitpoints didn't seem more valuable than immunities and/or slots for a caster IMO. Gifts from Heaven hit for 150, which is lethal to any mindless unit. Some enemy SCs were scripted to attack largest some killing high life enemies instantly, such as Micah's attack largest blitz with magic weapons. The Siege Golem loses both hand slots which also would have weakened my caster's powers further. That aside, I was tempted to go with a different unit who had more life despite the loss of immunities, but decided against it because of reason#2 below.
Reason#2... raise skeletons & the wraith crown sometimes generate skeletons with 15 life. The mechancial man has 8 (tied with imps) so he could sometimes be effectively hidden against the "Attack Largest" SCs and numerous spells that target the units with highest hitpoints. Why be the target of an enemy disintegrate or petrify or another spell if I didn't need to be? I didn't see painting a big bullseye on my caster as a good idea. The first 3 fights I was up against "Attack Largest" SCs.
That aside, I didn't expect for my SC to cast "Drain Life" so often which eventually did raise his life later above 20 at one point. I spent the death gems faster than expected and Potatoman's Robusta kept coming back from the night of the living dead so often I was doomed to fight Baalz's Poison Golem gemless. By the time I fought that Poison Golem I really needed 20 ancestors to delay that imp wave so I could get eventually get a disintegrate through, I only needed 1 disintegrate to work on his Poison Golem to win it all... but ran out of time. *Perhaps* a Siege Golem may have lasted long enough until Baalz fled, but would the same Siege Golem have won against the other 3 earlier fights with a weaker magic (more fatigue per spell, less penetration) and a big bullseye on him?
Finally, the 1st WitherBones was not only for anti-spamming, but I also saw it as my best shot to kill any 5-15hp lich in the 1st round due to the large AoE... before ultimately losing the imp/summon wars to them as my disintegrates would never find him due to his low life.
On another note, I think Taqwus had the worst luck... both times his SCs were killed before they could even do anything.
Wraithlord's Touch of Madness didn't work because that spell requires 3N and he had no nature gems.
I was impressed with many of the builds but this post is long enough so I won't elaborate. And since I did steal Baalz's double lifelong protection idea for this build I'm ok with losing to him... this time.