Tip overview

Buck has a great capability of healing himself. Deny his heal when possible

Try to fight fro mid or long range, so his shotgun does less damage

You can't outrun Buck, but you can easily outdamage him.

If Buck has Ensnare legendary and you're a prime target, Resilence can work better than Haven, due to huge damage buff while the slow effect lasts.

Avoid long duels vs Buck. Charge and burst him down or run. Fight Buck with a pillar or an obstacle between you and him and you will lose if he gets out of cauterize to heal up again.