What is happening neurochemically when we seek to reduce stress by taking aggressive action?
So there's a lot of things to talk about here.

First off, there are many strategies one can use to regulate stress.  James Gross, for example, has put forward an incredibly influential process model of the strategies we might use. His work and others have pointed to:


Distraction
Reappraisal (reinterpreting the situation)
Expressive suppression
Thought suppression (Wegner)
Rumination (Nolen-Hoeksema)
Worry
Self-distancing (Ayduk, Kross)
Situation selection
Situation modification
Interpersonal regulation (social support)
Acceptance (Linehan)
Exercise
Social sharing (Rimé)
Externalizing behavior (e.g., aggression)
Drugs
And more…


Not all of these are "effective," but you can see that there are a lot of ways to cope with stress.

When talking about aggression, there's likely a bunch of processes going on that could change depending on the context and individual.  The psychophysiology and neurochemical changes from one instance to another, and from one person to another, are likely not going to be the same. So there's not really one answer to your question.

Indeed, aggression:


can be negatively or positively valenced (i.e., it can feel good or bad; e.g., Chester &amp; DeWall, 2015)
might involve some sort of external or internal focus (recruiting relatively more exteroceptive attentional networks, e.g., Wager et al., 2015 or interoceptive cortex, e.g., Dambacher et al., 2015, respectively)
might involve a lot of top-down regulation (e.g., Achterberg et al., 2016) or be largely impulsive
might motivate approach or withdrawal behaviors


And this logic extends to neurotransmitters, neuropeptides, and hormones.  Their activity depends on the context and the person (over time).  In general, you can probably expect a complex interaction among these things, and involvement of practically all of them.

So as with most things in psychology, the answer is: it depends!