The Bugbear That Files a Repositioning Order
Read the stat block and it looks like a bruiser: AC 15, 65 hit points, a heavy mace and a javelin. Nothing there dictates the fight. The Stealth modifier of +6 does. This creature wants to open unseen, which means surprise, which means a full round where it acts and half your party does not.
The Abduct action is where the joke encounter stops being funny. On a hit, the target is grappled, and the Stalker can move at full speed carrying it. That is not damage, it is relocation. It pulls your wizard out of the back line and into the dark, or drags your cleric away from everyone who needed that Bless. Sixty-five hit points exist so it can afford to keep doing this while you scramble to catch up. Ignore the mobility and you lose the fight on positioning, not on numbers.
The counter is turn economy. It has one body and one Abduct per turn. Concentrate fire, drop it before it makes a second run, and the whole plan collapses into a corpse holding a mace.
At the table: When it grapples an ally and starts moving, ready no attacks. Break the grapple or kill the Stalker that round, because chasing it costs you the tempo it is buying.