Dark Souls 2 Boss: Demon of Song
The Demon of Song is basically a giant toad, but its heavy armour means you can only damage its face when it pulls back its 'hood' to reveal the creepiness beneath. Stay fairly close to prevent its ranged attacks, wait for the easily avoided arm attacks to end, then get a few hits in while it's still vulnerable.
Dark Souls 2 Boss: Giant Lord
When fighting the Giant Lord, try and hug his feet as much as possible - though watch out as he has a stamp attack, and some of his sword sweeps can also hit you even if you're slightly behind his legs. Stay to the edges of the arena, as you want to make sure you avoid the fireballs raining down!
Dark Souls 2 Boss: Guardian Dragon
The main thing you need to watch out for when fighting the Guardian Dragon is, unsurprisingly, its fire attacks. If you see it perching on a wall of the giant cage you're fighting in, run to the opposite side as the fire breath that follows has a wide area of effect. Keep dodging the fire, then get in to its feet while its on the ground to deliver a few big hits before it moves again.
Dark Souls 2 Boss: Velstadt, the Royal Aegis
To get started in this fight, bait Velstadt into destroying the pillars dotted around this thin arena, which will give you more room to work with. This enemy is fast and hard to circle strafe due to their sweeping attacks, so your best bet is backing off to avoid their big swing then dashing in for one or two hits maximum, before quickly backing off again as they follow up with another attack fast. About halfway through they imbue their mace with magic for extra damage, but keep doing what you're doing until you secure victory.
Dark Souls 2 Boss: Throne Watcher and Throne Defender
Here you're fighting two bosses at once, so summoning an NPC to join the fight will definitely help you out. The Throne Watcher is the faster, more agile enemy so it's worth taking them down first - but be aware that if you don't defeat both bosses within a short amount of time, one will revive the other. Tactically, you should aim to get one down to a third of their health or less, then take the other one out before finishing off the first boss.
Dark Souls 2 Boss: Nashandra (FINAL BOSS)
One of the main things to look out for in this final boss fight is the glowing curse orbs that Nashandra summons around them, as you'll get cursed by them whenever you get too close. Try to hold back and use ranged attacks to kite Nashandra away from the cursed area, so you can then get some hits in. Their beam attack is hard to dodge, so if you can't outrun it then at least get your shield up to minimise the damage. This fight isn't actually too tricky if you can get your head around that approach and mitigate any cursing you receive, so keep plugging away until you achieve victory.
Congratulations - you've beaten all of the bosses in Dark Souls 2!