Arena used to add 10% of your real stats. That was removed back when 2.0 went live in janurary last year. Arena no longer use any stat other than initiative.

Roshi is extremely squishy in arena due to the way damage formula scales, as well as lack of (hp) mood. The amount of damage ninjas deal and receive (as a percentage of their total hp) are so different in arena, when compared to two players at 100k power each, that arena might as well be a completely different game.

Mood is not applied in arena, all ninjas are 0% mood. Ninjas that heavily rely on mood for survival suffers extra in arena.

Roshi cannot tank in arena, at all. He can flat out die to mabui 1010 teams or mb blitz teams on round 1, AFTER han removes enemy buffs. At 100k pow, outside arena, that roshi will survive with half of his hp left after han's buff removal. Roshi simply never get to stack up in arena, unless your opponent is also running a very passive team.

All forms of life gain, be it heal or leech, being reduced to 50% power in arena certainly does not help roshi survive either. (With healing tips from water main, heals are at 90% power, instead of the 140% power outside).