This doesn't feel mythic to me. Compare the ability to Kyren Negotiation which was dude or player and was an uncommon. If she had haste and was dude or player, ehhh, maybe?

If I'm in Boros and I'm leaving guys back on turn 3, it's cause there are blockers in the way or I fear a crackback, so dude or player only makes sense.

Sure she turns summoning sick guys into strictly worse tims, but is that really what you want from a mythic?

Also, I'd point out that Negotiation was anytime.

I would change this to:

1RW Haste, First Strike Tap an untapped creature you control: ~ deals 1 damage to target creature or player 3/1

Activate this ability only if ~ is attacking.

If you keep her as a triggered ability, it is templated incorrectly. It should read "Whenever Arcos attacks, you may tap any number of untapped creatures you control. If you do, Arcos deals X damage to target creature or player, where X is the number of creatures tapped this way."

I do believe in this card, though, given the slight upgrade of haste and the ability to burn creatures.

Not really mythic - but solid. A 3/1 first striker for 3 is always a nice thing to have; and the ability to force through extra damage is a nice finisher later.

Not having haste, and so giving the opponent warning of the extra damage is also an important feature; making this answerable.

But still.. doesn't quite feel mythic to me.