I have been using X.Borg weeks ago on the advance server. In fact, I played with this hero so much I have grown tired of him lol.
Anyway, I have experimented with different item build for this flaming cyborg and found out the best balanced build for him.
First item - Blood Lust Axe.
X.Borg's skill is perfect for this item specially his first skill that damages enemies in a wide area allowing him to spell vamp lots of HP. Plus it gives you a cool down reduction.
Second item - Warrior Boots.
This will allow X.Borg to gain some physical defense.
Third Item - Wings of the Apocalypse Queen / Queen's Wings.
This will give you another 10% cool down reduction, 1000 hp and 15 physical attack. But the most important is its passive ability which enables you to reduce the damage taken by 50% when your hp drops to 40%. This will give you more chance for survival when you release your Firaga Armor after using your Ult.
Those are the the first three and basically the core of X.Borg's item build. I often play around and experiment with the next three but I always find myself settling for more damage.
Fourth Item - Blade of Despair.
This gives additional 170 physical attack and 5% movement speed but its real perk comes from its passive ability which increases your physical damage by 25% when you attack an enemy hero whose health is below 50%.
Fifth Item - Athena's Shield.
X.Borg is notorious for being a survivor because of his passive ability which is the Firaga Armor that regenerates itself. But he is vulnerable without it, so in order to increase his survival you need this item that grants a shield every 30 seconds.
Sixth Item - Blade of Despair or Glowing Wand.
Choose BOD if you want more physical damage and Glowing Wand if you want to add some burn effect with your flames that burns through enemies for 3 seconds.
But if your team does not have a tank, you should buy Brute Force or Cursed Helmet as the final item.
Here's a video explaining my
X.Borg item build guide with some gameplay.