The highest damage offhand weapon you can wield is the sanctified gladius, that means your melee attacks will have a fair amount of holy damage. There are other options though, like the Wing of Asira for Disarm immunity. Dual wielding means you'll get 2 attacks every time you make a weapon attack, including AOEs so you should definitely use Cleave. If you're gonna be weapon attacking a lot, Charge Weapon is good to add more damage to your weapon passively.
That leaves you one recharge slot, the encounter slot, and the ultimate slot. Those are pretty flexible and you can change them as you like, though weapon based powers will likely be best because they benefit from the dual wielding and the weapon buff from charge weapon. That said, bringing some kind of healing or crowd control is also a good idea. Personally I use Power Wave as my other recharge power, it adds a damage type and the attack is ranged to let you hit fliers and it recharges real quick while hitting hard. Dastardly Trick is a strong weapon attack that also stuns, good for breaking an enemy out of a dangerous stance. If you have spellpower though, Dawnbreak is even better hitting all enemies for big damage and giving you another weapon buff. Assassinate is a weapon attack and also useful for taking down dangerous targets quickly.
Companions are also pretty flexible depending on your stat spread, though you'll want strength if you're doing weapon attacks. Willpower, Agility, Cunning, and Toughness are also important stats but since you can only buff 4 you'll have to dump something. It's debatable which stat is best to dump so I'll just leave that to your discretion.
Personally I level Strength, Willpower, and Agility and if I remember to take a drink buff I use Cunning. Then I just take as much spellpower gear as I can. The real decision you'll have to make is whether you'll have a tank or not. There's some split on this so I can't really tell you which is better, I personally don't use any tank though I don't know if that's optimal or not. I will say you typically don't need a tank if you've got some real damage on your team even against bosses, just one healer is usually fine.
Yes this does just end up being basically a tweaked version of Cait and Arona's spellblade builds, but those builds are fantastic to the point that running both of them as spellblades along with this build is a perfectly viable strategy against like, 99% of bosses.