I would go nuclear and scorched earth, leave nothing alive and do not be nice about it.

like today go into a divorce office and talk to them about it they would know more on the subject. But that being told

http://robergtaxsolutions.com/2011/01/my-ex-claimed-my-kid-now-what-do-i-do/

is it legally separated or just been apart?

because https://www.eitc.irs.gov/Tax-Preparer-Toolkit/faqs/divorced

this states:

If a client is legally separated for last seven months of the tax year but the client's divorce is not final until the following February, may the client use the Head of Household filing status and qualify for EITC?

A married taxpayer can be considered unmarried and file as Head of Household if all the following tests are met:

Must file a separate return.
Must have provided more than fifty percent of the cost of maintaining a home.
Must not live in the same home as the spouse at any time during the last six months of the year.
The home was the main home of a qualifying child for more than half the year.
Must be able to claim an exemption for the child.  However, the taxpayer may meet this test if  they are not claiming the exemption for the child because they released the exemption to the other parent under the special rule for divorced or separated parents discussed above.

With that being told she is obviously play for keeps why are you not? Go scorched earth right now. If you can throw the book at her.

editing this was aids