But I heard that Thundra had a relationship with the Hulk. That level would be? What?
That would have been the one-shot issue
Hulk: Raging Thunder, which came out earlier this year.
However, its not the Thundra we've known, but a variation of her, one it seems that never sought out the Fantastic Four and has as a consequence retained her extreme Femazonian (Femanazi) view of men as not only inferior, but to be exterminated (genercide). In HULK: Ranging Thunder she didn't have a sexual relationship with him, but rather "took" a sample of his gamma infused DNA and blending it with her own, and "created" a daughter.
Her daughter, it remains to be seen if she'll retain this marshal philosophy of sexual-bigotry, or like the Thundra of 616, will gradually evolve into a more balanced individual.
The extent of her strength/invulnerability/endurance hasn't been stated yet, but appears to be markedly superior to that of her mother's. Whether or not its on a par with Jen's, I think remains to be seen, and which will depend on how Marvel plans to fit her in.