she does gain souls but the way it works is that it's based off the % of damage given to the opponent. Since their health pool is so large, it's impossible to keep the souls going well enough to play patiently.
If your goal is to blast him with an L2 and reverse the heal, you'll need to be aggressive, and build your L2 as quickly as possible. provoke attacks, keep your distance close, and get your 20 some hits in. after that you need to continue to be aggressive and proc as many heals on him as possible.
Also remember you start the fight with 4 souls. And each one expires after 10 seconds. This can be extended with the Morningstar synergy if you bring her too.
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If your goal is to blast him with an L2 and reverse the heal, you'll need to be aggressive, and build your L2 as quickly as possible. provoke attacks, keep your distance close, and get your 20 some hits in. after that you need to continue to be aggressive and proc as many heals on him as possible.
Don't use the despair mastery