How does one go about killing old king Doran?
There are a variety of ways. Have you gotten the sword already in the "proof of worth" duel? When you aggro him after that he gest WAY tougher. He is very easy to parry if you get the timing right, but the problem here is his HP, can heal, instakills (Unless you wear full Dark Silver and Shield alonh a bunch if HP), and the otrher mobs.
Firstly, clear the zone. Then, you have a variety of options. You can go in Hyper Mode and damage him normally-ish, critical him,
poison with the spells or the Kunai, or even bleed, use the Strom Ruler R2 to knock him off the shortcut tower, blow him up at the barrels there, summon people, etcetera.
The other thing to note is his aggro and healing. he doesn't go much further than the tower before the Mausoleum so taking him further requires to push him around, which is risky. That means that he is kinda hard to cheese with projectiles, and if he does heal, which isn't that often and punishable but a bummer nonetheless,it's a do-over and you spent some hefty ammo on it. At 33%HP he goes beast mode and two hands, not much change but the moves change a bit and it's an even more certain ohko.
What I did was pushing and backing away in the tower and catapult area, poisoning him with the Kunai from Graverobber Blige, and keeping constant watch on him to not heal, using a maxed Quality Knight Sword and the Dark Silver set that my equip burden could afford, of course the Dark Silver Shield for its buff, actual block and parrying constantly, around three times of poisoning him to keep him in the ailment, and bam, poof goes the fucker. Remember to come there light to be able to get away with the bounty.