The only thing more widely known than a bugbear’s tactical prowess is the fact that both bugs and bears love honey. Defeating this foe is as simple as placing a quart of honey (retail 50 copper pieces from most vendors) under a tree large enough to climb. Getting a surprise round on these beasts is all you need to guarantee a victory, and anyone possessing an ounce of acrobatics should be able to achieve a charge attack (with bonus) by leaping from a nearby tree.
Remember that Bugbears travel in clans, and often you will have to dispose of a scout and several soldiers. With clean enough blows, you can use the same honeypot on multiple enemies before settling into a full-fledged skirmish.