Throne Watcher and Defender
As a final step to unlock the last boss, you must defeat the Throne Watcher and Throne Defender. This is a gank boss. You will face two strong enemies at the same time. The fight is difficult, tedious, and time-consuming. The video below shows how to cheese the boss.
Dark Souls II: SOTFS - Throne Watcher And Throne Defender CHEESE + BREAK
Killing the boss via the cheese strategy can be equally time-consuming. On the other hand, you can summon Benhart of Jugo to distract both enemies while you spam Crystal Soul Spear.
Drangleic Castle
Start from the King's Gate bonfire. Attune any spells you need for the upcoming boss fight. Crystal Soul Spear is effective against the boss. Equip the King's Ring, head downstairs, and wait in front of the iron gate for it to open. Furthermore, equipping the King's Ring prevents the nearby Royal Guard from re-animating and attacking you. Proceed along the path ahead to reach the fog wall. The summon sign of Benhart of Jugo is nearby. Summon Benhart to help take some pressure off your character. Consume a Brightbug prior to summoning Benhart.
Throne Watcher and Throne Defender
Sorcerer & Benhart versus Throne Watcher & Defender NG+7, Dark Souls 2 SotFS
This is a gank boss. You will fight the Throne Watcher and Throne Defender at the same time. The two-versus-one fight can drag on for some time as you attempt to wait for an opening to attack. Summon Benhart to help take some pressure off your character and shorten the fight. Enter the boss arena, immediately lock onto the rightmost enemy, and spam Crystal Soul Spear. Cast four times, then roll diagonally to your right to dodge any incoming attacks. You want the Throne Watcher and Throne Defender to be distracted by Benhart while you spam Crystal Soul Spear from a distance. Cast Crystal Soul Spear a few times against one boss, then switch to the other boss. Their HPs should be approximately the same. When one boss is defeated, they do not die but kneel down waiting for the other boss to revive and heal them. If the HPs of both enemies are about equal, then you should be able to kill one boss and then the other within a short amount of time.
The fight is also a race against time. Benhart is not strong enough to tank the hits from both enemies, especially after the enemies buffed their weapons. Equip the Magic Clutch Ring and the Blue Dagger to boost the damage output of Crystal Soul Spear. The best opportunity to cast is when both enemies are aligned in a row so that Crystal Soul Spear would hit one enemy then the other. However, this ideal alignment can be difficult to come by because one of the enemies is quick and agile (i.e. the enemy in white armour).
Defeat the Throne Watcher and Throne Defender, send Benhart home if he is still alive, and warp back to the King's Gate bonfire. You want to be back at the King's Gate bonfire before the final fight is triggered. Alternatively, fight the last boss now.