He is a top tier arena champ. Not sure what use he would be in clan boss teams, maybe someone can correct me on that though.

I would think savage+cruel or crit damage would let him nuke pretty hard.

his first attack on clan boss then health damage attack on boss  does alot damage

rotos is god like in relentless set

I'd drop in Savage and Cruel. Nothing like ignoring 30% of a target's defense and then 90% on his A3.

I  think  the  ideal  build  for  Rotos  is  Relentless  +  Immortal.

The  problem is  I  think  it  is  very  hard  to  pull  off  the  above  set  up.

The  Best  Relentless  Gear  is  won  from  placing  number  1  in  tournaments.

The  Best  Immortal  Gear  is  acquired  from  UNM  Clan  Boss.

You  are  looking  at  6  Gear  Pieces  which  are  Non-Farmable.

Obviously,  A  lot  of  people can't get  the  Ideal  Build.

A lot  of  people are  forced  to  settle  with  something  else.

I  have  seen  people try  the  following gear  sets:

-  Savage

-  Cruel

-  Lifesteal

-  Regeneration

-  Stalwart

-  Accuracy

-  Perception

It  just  depends on  what  gear  you  have  +  what  you  plan  to  use  him  against.

Stats > Sets

All things being equal, for Tag Arena (especially defense) I would want relentlesss just for the extra rng procs.  I know I love it whenever I have 4 left against rotos at full health and he gets 17 turns in a row killing all.  LOL   

Rotos is almost completely useless against CB. Don't waste your time with him there. There are far better choices for CB teams - all he brings is DEF down, which a million other champs can provide. Also, his A1 extra turn mechanics will completely throw off your rotations.

As for arena - there's quite a few ways to build him. Throwing one out there for you to consider - all of his skills scale on HP as well as ATK, and they scale roughly proportionally. That means you can run him on a defensive team just as well as you can on a speed nuke one.

Personally - the team he's going on for me is my 3rd/4th string 3v3 team. Gorgorab lead, Umbral Enchantress speed tuned to go right after Gorg, then two of [Rotos/Royal Huntsman/Bloodgorged] depending on affinity. Since that team runs ATK up, I'll probably focus on gearing him for ATK. But he could just as easily fit on my DEF/RES lineups, since he can just stack HP gear and still put out crazy amounts of damage.