Yeah, the forum setting would definitely make this harder. Defend is a tricky little move, since it kind of lets you step into the middle of another move.
There's at least two cases here:
There's the softer move of "show signs of doom," where you say something like "the orc skullcrusher starts charging toward you, building momentum to swing his huge hammer, what do you do?" If, fictionally, someone's nearby who might be able to attack him, they might be able to, it really comes down to the fiction. If this was, say, the first the players have seen of this orc, they were just exploring until now, they can probably try to bring him down before any damage occurs. Defend definitely gets in the way, but a Hack and Slash might be appropriate too.
There's also the harder "Deal damage according to the threats present" move, which Hack and Slash can't do anything about, but Defend can. This is where you say something like "the skullcrusher heaves the hammer over his head, swinging it straight down onto your arm..." Defend specifically lets you step into the middle of that move and interrupt it, Hack and Slash doesn't let you do that.