Bristleback has been called "the tankiest hero in Dota 2," and while it might be true, it doesn't tell the whole story.
After this, your skill build should prioritize Quill Spray and the Bristleback passive, depending on if you are taking more damage. The Bristleback passive reduces all damage coming from his rear or sides, so this works wonders to increase effective health. Get your ultimate whenever possible, and make sure to squeeze in one point of Viscous Nasal Goo around level 4 or 5, as the skill excels with only one point in it.
The next build is a little more complex, as it focuses on dishing out magic damage and healing from it. The core item here is Radiance, an item which lets Bristleback pump out consistent damage throughout a fight as he stands in the middle of it and soaks up damage. The next big item to go for is Octarine Core, which among other things, gives plus 25 spell lifesteal. This works with both Radiance and Quill Spray, meaning you'll be healing for upwards of 200 health per second in a team fight.
Keep your back to the enemy whenever possible, even when chasing, be ready to turn on a dime to prevent 40 percent of damage taken. This means in fights you'll want to be smack dab in the middle with your back facing all five enemy heroes. If you do this with the right items and the right skills, your MMR will start stacking up faster than Quill Spray stacks.
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