From: New York, NY Registered: 2009-10-02 Posts: 458 Website
ただでさえ = under normal circumstances 常人ばなれ = far away from a normal person 頑丈さ = strength, sturdiness
"His strength which under normal circumstances is far superior to an ordinary person..."
I don't have the context but I'm not sure that "pick a fight" is the best interpretation of 絡む. (Even if it is, Ren is the subject, not the object, so he'd be the one picking the fight!) I would say, "If Ren gets involved" or something like that...
"But, he is Serisawa Tatsuya -- he's a man whose strength is far superior to an ordinary person even under normal circumstances, but if Ren gets involved it's rumored that it would surpass the human limit."