What is the meaning of 怼回去? I came across a comment:

巴西柔术被误以为女子防身术, 怎么怼回去

What is the meaning of 怎么怼回去?
怼 is fairly new internet slang.
MDBG defines it as:

怼
duǐ
(Internet slang) to attack verbally / to publicly criticize / to call out

notice: it is read in the third tone here and basically means to lay into.
With the rise of popularity of 怼, many new words came out and have started popping up everywhere.
Not to get off topic too much, but another common new word is 怒怼.
MDBG:

怒怼
nùduǐ
(Internet slang) to chastise / to angrily denounce

This hopefully gives a better overall view of what 怼 actually means.
Another new word, that would answer your question more clearly, that has come out is 回怼:
MDBG

回怼
huíduǐ
to retaliate verbally / to hit back (at critics)

So back to your question:

怎么怼回去?

Brazilian Jiu-Jitsu has been attacked and now they’re asking how to retaliate or how to hit back at their attackers.