Via studio you can enable "Skill"  many2many field to tree view.

Then group by "Skills"

Well you could create a computed field on the Employee Model. Assume skills are related by a skills_id field and that the Skill Model has a skill field titled name. Then reference this computed field in your view.

But this won't really be one skill per line. It's just all the skills on one line (similar to the tags). If you want one per line, you may have to write a Skill view which filters by your selected employee(s) or your the group-by as suggested by someone else.

skills_text = Fields.Text(compute="_compute_skills")
@api.depends('skill_ids')
    def _compute_skills(self):
        for record in self:
            record.skills_text = ", ".join([x.name for x in record.skills_ids])

Odoo default seems to be what you are looking for 

Probably you might need to filter (skills != ) and group by employee