In the first stage of the battle, Samael uses only one attack, but in several variants. He has an innate ability to teleport which he uses frequently to shift to the character's present location and hit it a few quick times. This means you need to be moving all the time and use dodging without a fail.
Luckily, the boss is vulnerable to all kinds of attacks, including the standard melee ones. You need to know, however, that it's harder to use the Death Grip method here. It works best when you can aim at the enemy, but since Samael disappears for a while every time he teleports, the aiming mechanism can get confused. Consequently, I recommend using Repear's Revenge and all of those special attacks which don't require aiming.
Keep hitting the boss until he goes back on his throne. He will then send three fireballs at you (one at first, then two simultaneously), but as long as you don't get your moves mixed up, you should avoid getting hit by any of them (they move in a straight line).
Get ready to resume the offensive as soon as you dodge all the fireballs. This stage of the battle isn't any different from the previous one, though the boss will probably start using more powerful attacks when he teleports to you. Don't stay in one place for too long and keep looking for opportunities to deal as much damage as you can.
Same as before, attack the boss until he goes back on the throne. This time Samael will send five fireballs your way (the second and the fourth from the left, the middle one, the first and the fifth from the right). Try to dodge them effectively so as not to lose any unnecessary health points.
As soon as the fireballs are out of the arena, Samael will resume his teleport attacks. Don't change anything in your offensive, just keep hitting him until he loses around half of his health. He should then retreat to his throne for longer. Run to him and attack (the above screen), triggering a cutscene during which you need to press E.
In the next stage, Samael will stick to teleporting around the battlefield as well, but this time there are few additional difficulties to take into consideration. The first one are yellow glyphs which will be following Death on the floor (the above screen), and to avoid getting hurt you'll need to run away and time your dodges precisely.
The second difficulty is that the glyphs will eventually show up on the whole arena. Keep moving constantly, but watch out where you're going and if you're not stepping into one of the glyphs.
Due to the fact that Samael had become much more resilient since the second stage of the battle, it would be good idea to use the Reaper Form. Apart from that, make use of the most powerful special attacks and don't be afraid to use curatives to restore the Wrath bar.
Keep hitting Samael until he runs out of health. This will trigger a new cutscene and conclude the battle, rewarding you with a Demon Key.