BOSS: SMELTER DEMON
The first boss of Iron Keep is quick and fierce, but can be easily matched if you have a human effigy to spare. This will gives access to the phantom known as Lucatiel. While having a phantom will increase the smelter demon's HP, Lucatiel is an invaluable bait that can keep the demon occupied while you pummel at his life bar with ranged weapons. When taking him on up close, be mindful of the demon's heat core charge-up move, which deals fire damage when you stand close to him. The demon's stab is especially harmful and tough to dodge, but it is nonetheless blockable. All the flames can feel overwhelming and intimidating, especially when the smelter demon uses his slash combo. That said, you do have a good chance of avoiding many of these attacks with a classic dodge roll toward the demon. This will often position the demon with his back to you, creating an ideal opening to attack.
THE SPIRITS OF BELFRY SOL
You can find Belfry Sol at the top of Iron Keep. It's a decent area to gain some levels and vanquish a number of dark spirits. Why they decided to congregate here is anyone's guess, especially when the area's balance of open space and cover points works to your advantage. These dark spirits also make for an amusing variety pack of opponents, from the spellcasting bell keeper mage to the greatsword bell keeper to the dual-crossbow-wielding dual avelyn bell keeper. You should also be on the lookout for the area's only actual invader: another pint-size blood-red bell keeper.
BOSS: OLD IRON KING
This duel versus the Old Iron King in Iron Keep is one of the best tests of patience in Dark Souls II. While this demon is relatively slow in his attacks, do not be fooled into thinking that you can pummel him with melee blows and long-range spells any more frequently than other bosses. The time that you'd otherwise spend on an additional attack could and should be spent recouping any HP you've lost. Study how long it takes the Old Iron King to finish a move so you know when you can take a swig from your estus flask. You might realize that you're better off repositioning yourself before you rush to heal. The good thing is that his non-fire attacks are notably short in reach. And it's of little surprise that his ranged flame breath move will encompass either a wide area or a narrow one. If it's the more sweeping of the two attacks, you can actually roll in between the balls of flame without injury.
You'll also need a keen sense of spatial awareness because getting fatally knocked off into lava is as likely as dying from zero HP. Pay special attention to the hole in the center of the platform and try to avoid getting knocked into it (especially if your poise stat is low). One of the safer zones is at the end of the platform to the Old Iron King's left. Many of his moves can't reach your there; just be mindful that if he does connect, you might fall backward into the lava.