I can never understand why attacks by "activists" do not result in dozens of arrests and convictions for multiple years of prison every time they happen. Isn't it basically what any sovereign law says?
[0] Which may happen if the state thinks it cannot prove the allegation or the act was so minor, it would be a waste of everybody's time. But the "case dropped because too minor" doesn't usually happen in assaults, but more for non-violent things like riding the bus without a valid ticket or illegally downloading a few songs.
If you slap somebody, you'll likely get a fine, which is issued as "so and so many day's rates" (Tagessätze), which means e.g. you can get a fine of "20 day's rates" i.e. you'll have to pay the equivalent of 20 days worth of income, calculated based on your monthly net income divided by 30 days and capped at 1 EUR to 30000 EUR per day.
A prison sentence can be issued as a suspended sentence if it is 24 months or less, which is up to the discretion of the judge(s). You only have to go to prison if you receive another prison sentence for another crime in a time frame specified by the judge, or if you violate any of the additional requirements a judge may issue. If I remember correctly about 70-80% of all prison sentences below 24 months are issued as suspended sentences, and only a tiny fraction of people end up going to prison because the suspension is lifted later for some reason.
However realistically even the "dangerous" variant quite often ends in a light sentence of a couple of months which is then usually issued as a suspended sentence. This depends on what happened, the level of involvement and previous criminal history if any and whether you confess and show remorse. However, the suspension of a sentence is usually contiguous on you paying everything you owe, from damages for pain and suffering to the court fees.
You can't just get away with attacking people.
(Actually, on a serious note, the person going there with the sole goal of stirring up the protest is more likely the one to be the terrorist, no? I'd say taking away that sign and even potentially removing the person is actually de-escalating. But you should obviously let the police do that.)