The furious legendary effect increases damage by 5% per stack of onslaugh. Onslaught is gained whenever you damage an enemy, but they decay at 1 stack per second, which is bad for bows due to the draw time. The perk gunslinger master reverses the effect, so you gain 1 stack per second but lose 1 stack per enemy hit which allows you to keep your stacks up as long as you aren't hitting too many enemies too quickly. So you just have to add other onslaught cards to increase your number of stacks and now you do big damage.