Diarmuid and Artoria, the King Arthur, were both legendary warriors hardened by countless battles. There was no need to say more about the legend of King Arthur—her fame was simply enormous. As for Diarmuid, known as peerless throughout the world, his legends were hardly inferior to those of King Arthur.
Historically, he was the foremost warrior of Ireland's Knights of Fianna. Strictly speaking, rather than calling Diarmuid a spearman, it would be more fitting to call him a swordsman.
Strictly speaking, the two Noble Phantasms Gae Buidhe and Gae Bolg were the simplest weapons among those Diarmuid wielded.
Though their abilities were indeed formidable, once the opponent knew the effects of the two Noble Phantasms, their effectiveness became greatly diminished. If Diarmuid had not tricked Artoria, he could never have wounded her with Gae Buidhe.
King Arthur's combat ability and strength were not something Diarmuid, with nothing more than Mind's Eye, could compare to. In FSN, did anyone really think cutting down the heroic Heracles and taking seven of his lives with one sword was a joke?
The demonic swords Mor (Rage of Madness) and Beag (Wrathful Rage) were Diarmuid's signature Noble Phantasms. Putting aside the demonic sword Wrathful Rage for now, legend had it that Diarmuid used Rage of Madness to slaughter four thousand warriors of the Land of Fairy.
Strictly speaking, the added trait of Wrathful Rage should have been destruction. Combined with Gae Buidhe, its effect was simply outstanding. Unfortunately, Diarmuid, appearing as a Lancer, had not brought those two weapons with him, so they were utterly useless.
Incidentally, Artoria, as King Arthur, was not currently carrying the Sword in the Stone (King's Sword: Broken), the Holy Grail (Jesus's holy relic), or Avalon, which was with Kiritsugu.
Otherwise, a battle between Artoria and Diarmuid at full strength