Keeping fights interesting
Fights should be exciting, dynamic, and tense. The players should never feel like they're just "trading blows" or trying to deplete a foe's HP. Players shouldn't get bored waiting for "their turn," and the outcome should never be entirely certain.
Make soft GM moves all the damn time.
After every player move, describe the situation and make a soft move: say something that provokes action or raises the tension. Then ask someone, "What do you do?" Sometimes, your soft move will be offering an opportunity for a PC to act freely, or to follow up on a previous move's success. It's easy to do this accidentally, though, which results in the monsters seeming to just stand there and not do anything. Be intentional!
Make your moves—especially your monster attacks—colorful, descriptive, and specific. Don't just say, "It attacks you," say "It swoops down at you, talons out like this, coming right at your face! What do you do?" Demand the same of the players. If they say "I stab it with my spear," then reply with "Okay, cool, what's that look like?" If the player seems uncertain, or hesitant to commit, then offer them choices. "Are you, like, running at it? Bracing yourself? Are you going for its gut or its wings or its face, or what?" If you can't visualize the action, ask for more detail.
Consider the momentum of the action and other elements of fictional positioning. Incorporate them into your descriptions and your GM moves. So if your soft move was "it swoops at your face, talons out" and their response was "I brace myself and drive my long spear into its gut!" and they get a 7-9 to Clash, then the results should take all that into account. "Yeah, you impale it, but its momentum yanks the spear out of your hands and they both tumble over here, next to the edge. Roll your damage!"
Vary your GM moves, especially when a monster's attack lands. Don't always hurt them or put them in a spot. Lean on your monster moves to add variety. Sometimes, look at your list of GM moves and pick something you haven't done in a while, just to keep things fresh. No matter what move you choose: if it involves roughing up the PCs, or it could maybe take them out of the fight, then deal damage along with the move.
When a PC "whiffs" a damage roll, respect the fiction of their attack and the fact that they (probably) got a 7+ on whatever move allowed them to deal damage in the first place. The low damage roll means that it wasn't a telling blow, but it should still change the fiction. "Only 2 damage? Yikes, you don't pierce its hide. But it's still disoriented a bit, and struggling for breath—looks like you knocked the wind out of it."
Keep in mind what other enemies are up to. The PC stabbed the monster that was swooping at him, but what are its friends doing? Make a soft GM move to bring them into the action. "…looks like you knocked the wind out of it. Meanwhile, the next one is swooping down at you while you're unarmed, what do you do?"
Keep the spotlight moving. When one PC isn't actively engaged, make a soft move at the current PC, then move the spotlight to the idle PC and ask what they do about it. "…the next one is swooping down at you while you're unarmed. Rhianna, you see this happening, what do you do?"
Give less combat-oriented characters opportunities to shine. Encourage the Seeker to Know Things about the foe's vulnerabilities. Include spirits or beasts that the Blessed can interact with. Include victims to rescue, fragile treasures to protect, and puzzles to solve. Not always, and not every fight, but often enough to keep everyone engaged.
Incorporate the environment into your description and your moves: lighting, terrain, visibility, and the fog of war. They're on a massive staircase? Use a monster's attack to put them in a spot and bowl them half-over the edge. It's pitch black except for their sphere of torchlight? Have the monsters retreat and then announce trouble, "You can't see them, but you hear their calls as they circle for another attack, what do you do?"
Populate your battlefields with potential energy—heavy things to knock over, high places to fall off of, kindling to set aflame. Throw in some active hazards, too: raging fires, crumbling ceilings, sucking mire. Present these elements as opportunities to the PCs. Use them to answer "What here is useful or valuable to me?" Incorporate them into your GM moves, soft and hard.
Regularly recap the situation, especially as you move the spotlight around. "Okay, so: Vahid's on the upper part of the stairs. His spear and the creature he wounded are over here, near the edge. Caradoc, you're hanging off that same ledge. Blodwen, Rhianna, and Andras are on the other side of the gap, with Blodwen holding the torch aloft and Rhianna and Andras scanning the skies. Caradoc, you feel your grip starting slip. What do you do all do?"
End fights earlier rather than later. If it's clear that the PCs will win, have the bad guys flee or surrender. If things start to drag and someone rolls a 6-, use your hard move to drastically change the situation—a new monster shows up, a PC gets captured and dragged off, a follower sacrifices themselves to save the PC and finishes off the monster. If the PCs want to flee, let them Struggle as One to escape instead of playing out individual actions.
Finally, remember that this isn't easy. GMing is a practice, not something you master. You'll likely start out forgetting to do all of this stuff, and that's fine. Reflect a little bit after each session on what was fun and exciting about your fights, and what seemed to drag. Ask yourself what you could have done differently, and try to do that next time. Keep pushing yourself to improve!