There are five things you need to beat the Diablo Clone. You need to be able to survive his melee attacks, you need to be able to survive his spells, you need to be able to hit him, you need to stop his regeneration, and you need to be able to do massive amounts of damage. This means maxed blocking (defense is worthless here), maxed resists, smite or maxed conviction, prevent monster heal or open wounds, and crushing blow.
Equipment is straightforward. The two most important pieces of equipment are a Black runeword weapon (preferably a knout or scourge for the attack speed, but I use a jagged star), and a paladin shield with automod resistances and four diamonds. Other sources of crushing blow are good (I use Goblin Toes and Guillauime’s Face (Orphan’s Call), for a round 100% chance), and you need something with open wounds or PMH (Malice runeword on switch or blood-crafted belt are either one sufficient, and easy to get). A Dwarf Star is also handy, but not essential. The rest of your equipment is pretty free.
For stats, you want enough strength to wear your equipment, enough dex for maxed blocking (this will cost about two points per level, in the long run), and all the rest in vitality.
Skills, I’m less sure about: I know of at least one set that works, but I think I might be able to do it more efficiently. I put 16 into Resist Fire, just for the passive increase to max resist, but in retrospect, I’m not sure that’s necessary: 75% fire resist seems to be enough. You could also increase maximum resistance from equipment. 3-5 points in Holy Shield are essential, but after that, the bonus to blocking (the main reason we want Holy Shield) starts seeing some steep diminishing returns. I use maxed Conviction along with low-level Zeal to enable me to hit (-90% or more to monster defense, which is equivalent to +900% or more to attack rating), but a single point of Smite will also do the job, and you can combine it with the aura of your choice. On the other hand, Conviction will greatly help any party members who do elemental damage, since the Clone has such high resistances. And, of course, you want your standard smattering of paladin utility skills, like Cleansing, Meditation, Holy Bolt, Vigor, and Salvation.