It was a preemptive comment, seeing as how you obviously think Deadpool will win hands-down.
IMO, Archer can win this, because of how his Gae Bolg works.
From the TYPE-MOON Wiki:
"This single person attack (The Spear of Impaling Barbed Death), created by Lancer to suit his own style, is a fatal blow that always pierces the opponent's heart and ruins their body with its thousand iron thorns."
We're not talking about a simple stab or bifurcation here. We're talking about Deadpool's body being completely ruined. Deadpool has survived with only a head before (as you claimed), but can he survive having his entire body ruined?
And if you're going to say that Deadpool can just dodge,
"Once Gáe Bolg's name has been called, the cursed spear reverses the nature of causality to make it so the cause of the "lance being thrust" comes from the effect of the "opponent's heart being pierced" by it. The actual action of the thrust is merely a formality, as the lance has already pierced the opponent's heart before the attack has even started."
"It is impossible to dodge the thrust of the lance, no matter how agile, well guarded, or far away from the lance the opponent is standing."
That means that Gae Bolg does not miss, and Deadpool will be hit and will have his body ruined.
If you say that Deadpool's regenerative ability is too awesome,
"Once the heart has been successfully pierced, either by a normal thrust or its ability, it is nearly impossible to heal the wound with normal means unless Gáe Bolg is destroyed or Lancer is killed. The attack delivers damage equal to the opponent's own health limit plus the damage from the spear itself, so the wound will be fatal no matter the opponent once they are struck."
Deadpool cannot heal himself from a Gae Bolg wound.
Evidence for comparison:
"If the attack were to hit Arcueid Brunestud, who is capable of regenerating from a pile of cut up flesh, she would be killed as long as she wasn't saved by her luck."
The only way to defend against Gae Bolg is this:
"The only way to effectively defend against the attack is to have a high enough degree of luck to be able to alter fate, which would be heightened by also having good reflexive instincts, an auto-resurrection ability, or to have a shield"
Deadpool has none of those. If you bring fourth wall-breaking into play, I will call bullshit.
So I think Archer will win.