You and onli are talking about different situations.
At the low end of the spectrum, onli's advice is correct. At that end, trying to call in authorities is useless and fighting back is highly effective; even if you lose the fight, it'll probably convince the bully that picking on you is more trouble than it's worth.
Further up the spectrum where the power disparity is too extreme or the level of violence is life-threatening, yes, fighting back becomes less of a good strategy - it's still better than just sitting there and taking it, but better still is, as you say, seeking backup - and if that fails, leave the situation permanently.
At the low end of the spectrum, onli's advice is correct. At that end, trying to call in authorities is useless and fighting back is highly effective; even if you lose the fight, it'll probably convince the bully that picking on you is more trouble than it's worth.
Further up the spectrum where the power disparity is too extreme or the level of violence is life-threatening, yes, fighting back becomes less of a good strategy - it's still better than just sitting there and taking it, but better still is, as you say, seeking backup - and if that fails, leave the situation permanently.