These are just some tips about how to deal with the DLCs in DS2. Whether or not to summon NPC to battle, such as other tips on how to deal with bosses. This is very subjective and depends on players´ playstyle, really, so use it at your own risk.
Dont summon NPCs, except in some particular cases while some fights can be made easy by having NPC summons with you, my DLC experience (even in NG) is that they die too fast, you are left alone with the boss anyway, plus the boss has increased stats. I dont know who made it, but the stats increase in bosses feel much higher in the DLC.
Ivory king - fighting with one summon, my AR was 200-250 with Drakeblood Greatsword. Without NPC summons, my AR was above 500, so I killed the boss fairly easily. In this fight, the summons INCREASE the challenge and make it HARDER. Make sure you save all Loyce Knights and you dont need to summon anyone.
Aava - people will disagree with me here, but bring both NPC, really. I defeated her with 150,000 SM, and although the summons boost her resistances, who cares, they keep the cat distracted while you take her out.
Two nasty cats in fridgid outskirts - no freaking idea. I always bring NPCs here, two prayers, my grandma´s blessing and a pot of catnip and pray for the best. This is the only fight in this game for which I have no clue what to do. I beat them once in NG+ and with that, I didnt care anymore xD
On the Sunken King DLC, for the dragon fight - you can summon the NPC that has the dragon armor. He is almost imortal during that fight. The fight dragged on for me and lasted a while, because the boss kept flying. And the NPC was healing himself and had more than half HP at the end anyway.
Squalid Queen - although NPC boosts her resistance to boot, I do recommend having one here. Not sure which of them is best, to be honest. When she summons skelletons, you can use the Alluring skull to distract them, and that is easily dealt with. But Velstadt can be an issue. Having an NPC allows you to deal with him, while she is distracted. However, it wont work every time, and she will try to take you out with pyromancies. So I guess NPC or not is quite up to you, here.
Gank Squad - irrelevant. You will have to run around anyway, since there are three of them. I dont recommend bringing more than 1 NPC, because the Havel Set dude will be on steroids. Besides that, it makes little difference. They will probably swarm the NPC and take him out while you take out Robin Hood. Then you are left with the turtle and the katana dude, with boosted stats. Not sure if worth it at all. Another player is totally recommended though.
Sir Alonne fight - summoning NPCs is suicide. It makes his movements hard to track and he dispaches any NPC in no time. Then you are left alone with a hard boss with boosted stats. The area before him is basically like Iron Keep. With some planning, you can easily kill the enemies one by one.
Fume Knight - is a hard boss fight. Summoned NPC usually lasts me until his HP is around 30 percent. Then I am on my own. So I dont think it is worth summoning here either. The stat increase he seems to get makes the fight much harder, plus the NPC available does not deal such damage to compensate. This fight is better done on your own.
Old Smelter Demon - NPCs make the are before the boss easier, but unless you want to loot the area, just dash through and ignore the monsters as much as possible. Specially right before the fog gate. Summoning NPC here only makes the fight harder and the Demon trashes them as soon as his sword gets boosted. Also, your AR reduction caused by the NPC makes the fight drag on.
Armor and Shield
Using Heavy Shields didnt work for me, to be honest. Even the Havels shield+5 + Baneful Bird Ring + 30 stamina werent enough to properly tank against all bosses. Fume Knight´s attacks hit hard through the shield, even with high fire defense. Sir Alonne´s attacks always deal some damage, which initialy does not seem like much, but after some time into the fight starts making a difference.
Also, using heavy armors prevent you from being one-shot by the bosses (yes Fume Knight, I am thinking about you). But even at high vitality, the burden caused by the armor + the decrease in stamina recovery do not make it worth it. My experience is that, generally speaking, instead of 3 attacks, you can sustain 5 or 6. And while this sounds like a lot, in practice it is not. I think the idea is to avoid damage altogether. If they can´t hit you, it doesnt matter if you are using armor at all.
I had a better experience fighting the bosses with almost no armor (less than 25percent of total weight) and a small shield (Watchdragon Parma is very balanced and worked perfectly).
What Kind of Weapon to Use?
This is a sensitive topic. Every player has their own play style and something that works wonders for me, might not work as well for other players (duh). I tried many different kinds of weapons during the DLC, until I found a strategy that worked for me.
Heavy hitting weapons, like the Fume Greatsword, can deal incredibly high damage. But they take a huge toll on your stamina. Not only you spend a lot of stamina while swinging, the fact that they are heavy means that you have more weight, move slower and regain stamina slower (or so I think).
I used Crypt Greatsword for a while and the fealing of consistently dealing 600+ damage every swing is really good. Until you go to a boss fight, that is. In between boss attacks, you can safely strike once with a very heavy weapon, and deal high damage. In some cases you can strike more (when the boss is buffing a weapon, or is in poise break stance, etc.). But generally more than once felt risky to me. On the other hand, using something as simple as a rapier, or espada ropera, you can strike 2-3 times. With the right rings, you can deal 300-500 damage per strike. So although they have less AR, they end up dealing higher DPS.
Lighter weapons dont poise break as the heavy ones, so for the mobs they can be trickier. In my case, I found something in between. Drakeblood greatsword. Moderately fast, specially its 1H R2 attacks. It has very high counter damage, that can be further boosted with Old Leo Ring, to consistently hit 500+ in NG against Ivory King, for example.
And Those Tricky Areas?
Some areas are nasty to clear, like the fountain square in the Ivory King DLC or the water area with the giant frogs in the Sunken King DLC. However, the game rewards you for being carefull and thinking each area through, before acting.
There is a particular timing for the giant water frog area. If you go to the right side immediately after entering, and carefully follow the monster that moves away, you can take the loot from the dead body. Start moving back and you hear him rage quit, as he is able to see you. Run around the wall to your right side and you have an open path to the bonfire. At the bonfire, light it and rest. Get up immediatly and start running towards the exit through the middle area. You wont be bothered by the monsters and can even lift the loot from the dead body there, before leaving the area. You will hear one monster behind you rage, as you take the loot, but ignore him and run. He does not catch up to you.
Most other areas are easily cleared with some planning. Avoid awakening the golems, by killing everything away from them. Use a Bow to aggro the mobs and deal with them one by one - this works almost everywhere.
Finally, know the weapon you are using. Most of the monsters have decent poise in the DLC and going head-on against mobs is almost suicide. However, if you find yourself in such situation, try to get some backstabs. While you are not invulnerable during the backstab animation, you take much less damage and deal a lot in return. It is better than being interrupted in the midde of a regular attack.