Bristleback

If you can be aggressive, take a point in Viscous Nasal Goo as it will greatly improve physical damage output.

Turn your back to opponents, their towers or stacks of neutral creeps whenever you expect to take loads of damage.

Alert your supports to stack for you, especially ancient camp as you can clear it at level 7.

Apply aggression on the map as Bristleback by taking and defending towers, as well as rotating through portals.

Check opponents' inventories frequently in search for break effects and healing reductions. Black King Bar and dispel items counter some of them.

You can take Roshan down fast and early with your team thanks to Viscous Nasal Goo.

Use the Bloodstone active together with Bristleback active with Aghanims Scepter to do a massive burst of damage while healing yourself.

Magic Stick and Wand are must items against Bristleback's Quill Spray spam.

Someone should buy Spirit Vessel against Bristleback to counter his regeneration and healing.

If you have no good way of dealing with Bristleback, leave and don't waste too many resources on him.

Silver Edge allows you to break Bristleeback's passives.

Armor items are good to counter Bristleback's physical damage output.