While anything's possible, what about"old woman" makes you think tank?
To be honest, I don't look at people who are younger and much bigger than me and think 'that guy could take a hundred more bullets to the face than I could!', by the time we are talking about units able to bounce huge amounts of damage, technology and magic seems to solve a lot of fantasy issues. Any normal person in an overwatch arena, whether they are a 20-something bodybuilder or a 70-something lady, isn't going to last long unless they have something up their sleeve.
You could have an older magic user generating wave after wave of magical shields, or she could be piloting a mechanical suit, or have a summoned minion that is the 'tank' while she sits on its shoulder or whatever. 'Tank' classes don't just have to be huge and physically tough, it's that they can aggro and absorb damage that counts, not how they do it.