Kalameet boasts huge HP pool and his attacks do tremendous damage. As if that is not enough he is also highly resistant to all types of damage and I think he's also completely immune to all Aux effects, like Poison and Bleed. However, while he doesn't have a weakness to Lightning like all dragonkin should have, he also doesn't have immunity to Fire, thus you can use Pyromancies to damage him.
Here are some tips on how to beat him:
• Your best bet is to roll through his attacks as he can get through your Stamina rather quickly even with the best Greatshields.
• His breath does Magic Damage, so use shields with high Magic Defence. However the damage is somehow split, so you can't rely on your shield alone, nor on GMB to block all of the damage.
• It's best to evade Kalameet's breath and his melee attacks in general. However you can block his attacks. You will get guard broken, but Kalameet is far too slow to attack you, so don't be afraid to block his melee, or long range breath attacks (given you have high Mag Resistant shield). I always use Crest Shield for Kalameet. I can block all of his attacks with it and receive minimum damage. It's good for the situations where something screws up and you can't evade.
• Only attacks you have to avoid is his 360 Flying Breath and his Telekinetic Attack. Both are telegraphed with a lot of time to evade. Flying Breath starts by Kalameet flying directly up, that's your cue to run away. It's the same attack Hellkite in Undead Parish does. Telekinetic Attack starts by Kalameet standing on his hind legs. He then either breathes fire, or uses this attack. If you stand in front of Kalameet he will lift you to air. This does little damage, but it will inflict you with incurable status, which doubles all damage you receive. It looks like a small eye of fire floating above your head. The status will disappear after some time (30 seconds maybe...I never bothered to count, sorry).
• While Kalameet is resitant to Magic and Fire, you can still spam Sorceries, Miracles and Pyromancies to make the fight significantly easier. Especially all Spear spells are useful. Kalameet seems especially resistant to Dark Sorceries, so Pursuers and Dark Bead will do you little good here.
• You can land critical hits for huge damage if you manage to hit Kalameet's eye with a arrow. It will also stagger him, no matter what he does and can force him to land if he's trying to use one of his flying attacks.
• The most important thing for this fight is patience. It will be a long fight, even with two skilled phantoms at your side, so don't try to rush it. You're only gonna get your team killed if you'll be impatient.
And finally, some tips for cutting his ugly tail off:
• A weapon with long reach and vertical attack. For example Scythe, or Black Knight Halberd. Both are long enough to reach Kalameet's tail even when he has it raised.
• Manually aimed spells with AoE effects, like Wrath of the Gods, Great Fireball and even Spear spells. You can aim these and other spells manually to hit Kalameet's tail whenever you get into the spells range.
• Kalameet uses some attacks and lowers his tail, which enables you to attack it with any melee weapon. Whenever you see Kalameet stand on his hind legs, sprint around him to his back and strike his tail. Also, if you, or your phantoms keep on Kalameets back long enough, he will turn his head to look behind him and try to squash you with his tail. You can use this to attack it, if you're fast enough.
I hope this helps you.