To defeat the goblin boss, start the fight by moving to the first indentation; wait for the boss and land eight hits—but you have to time it carefully to trigger the knockback effect, and you cannot move at all while doing so. Next, move as far left as possible and wait for him to approach. Once he does, he’ll charge; jump over him with a long leap and land another seven hits.
Then, head to the right side of the map so that a goblin assassin and a spear-wielding goblin drop down; attack them and take a hit from the spear goblin (heal up afterward).
Kill them, and another goblin assassin drops down—taking him out is a mere formality.
However, the boss becomes much harder because he gains a jump attack (you must strike at the perfect moment to trigger the knockback). Attack him immediately (eight hits), then move slightly toward the other side of the map; once a small amount of energy regenerates, hit him twice. Retreat to the edge of the map again, wait for him to wind up for a charge, and then dash.
Enemies drop down again, so repeat the same tactic as before. Here comes the tricky part: go to the left side of the map so the goblin mage drops first, followed immediately by the hobgoblin. Deal with the mage first, then try to take out the hobgoblin—though it’s not that simple, as a goblin assassin and a spear goblin will also drop down (if you haven't killed the hobgoblin yet, it becomes a bigger problem because you'll have to jump or dash frequently). But if you managed that, you know what to do ;)