I have watched the video, unfortunately. It is always important to remember that most often the videos begin after the initial confrontation or incident, when bystanders or participants are now aware something noteworthy is actually happening, so the full context can be lost which can give a false understanding of the event.
That qualification aside, the officer in question, no matter the context not captured by the video, was using unnecessary force that could very much be characterised as police brutality and would easily meet the conditions for prosecution for ABH, if not GBH, and the officer should not only be suspended pending investigation, but should be sacked and prosecuted.
At no time is it lawful use of force to kick or stomp the head of a clearly incapacitated person lying prone facedown on the ground, nor should a police officer use the butt of their taser to strike a person’s face and head whilst they are being restrained by another officer.
My take from reading about the incident and watching the video, is the officer went mad after one of his female colleagues was assaulted (it’s unclear if it was initially physically, but it could have been), went in to misogynist white knight mode and lost complete control, taking out his rage (fuelled by adrenaline) on both the person that may have initiated the altercation and those that were with that person (that seemingly were not actually directly involved with the initial altercation).
But, ultimately, it does not matter the reason behind such actions—it is brutality, no matter what proceeded it, as the police are meant to (and are supposedly trained to) stay calm in these high pressure situations, maintain restraint to ensure they are acting lawfully, ensure public order, and protect the safety and rights of those they are detaining, within reason.
He did none of those things, so deserves to be held accountable for his unlawful actions.