This b***h... is strong. Regeneration and super strength; She's basically Hulk. Wait, did I say basically? Actually, she's literally Hulk. She said she can't die in battle and she proves it time and time again. Unless you cut her head off, or stab her in her brain, she literally won't be killed (and it's still speculation assuming that will kill her). Since her limits have never been truly pushed, it's uncertan whether she can regenerate her arm but, seeing as madara proved something like that is possible, I will go with tsunade's word that she can regenerate anything. Her pseudo immortality is one thing, but her ribcage-susano'o busting, Bunta-sword lifting, ground-shattering strength is another. Strength like that puts down just about everyone and would even potentially cut through gaara's sand like a knife through hot butter. Very Very few people stand a shot at defeating her and she's extremely underrated. One thing people fail to realize is that she's a 5 in taijutsu. That means that even people like guy(without using gates) is going to par her in a taijutsu battle OR marginally outmatch her.
Her massive summoning, Katsuya, is her final deadly weapon. At first glance, katsuya doesn't seem like much, but she can provide vital support in the form of sensing, defense and deadly attack(acid). For that simply matter, it's already difficult to fight large summonings with just yourself... Only few people can do it effectively, especially while fighting someone else.
*NB: I've moved her up this her solely because of madara's regen feats. Otherwise, she's 1 below Mei. This position implies what she said is true and she can regen at least somewhat like what madara was doing.*