Ok, now see the list I used in testing:
Legacy Pyromancer AscensionMana (18)
4 Scalding Tarn
1 Polluted Delta
1 Flooded Strand
2 Volcanic Island
4 Gitaxian Probe
4 Rite of Flame
4 Pyromancer Ascension
4 Peer Through Depths
This list focuses on getting Pyromancer Ascension online as soon as possible. You'll easily find Pyromancer Ascension with the best 12 one mana cantrips in Legacy. There's a little protection in Remand and Force of Will which can replace either Impulse or Peer Through Depths or a mixture of the two. Two Grapeshot also makes the play-a-lotta-cantrips-double-Grapeshot-kill possible.
Critical Spells in Pyromancer AscensionThis may come as a big surprise to those without experience piloting the deck, but the most important spells in this deck are other than its namesake are Gitaxian Probe and Manamorphose.
Gitaxian Probe cuts a full turn off the deck's clock and allows it to combo off earliest turn 3. In the world of Legacy, that's huge! Just take a look at sample plays in this article and you'll see 2 turn 3 wins each beginning with a turn 1 Probe + cantrip, leading into turn 2 Ascension, and turn 3 Probe + cantrip + a whole lotta nonsense.
Without Gitaxian Probe, you'll only be able to combo off turn 4 or slower. That's the difference between winning or losing to Legacy aggro decks!
Manamorphose acts in a whole different way from Gitaxian Probe in that it gives essential mana while the deck is comboing off. The deck is essentially mana + cantrips and Manamorphose acts as both after you've ascended. It works in conjunction with Rite of Flame to give you 4 blue mana to add 4 storm count with your cantrips. This, and not Rite of Flame, is often the card that you'll be searching for after an active Ascension.
Speaking of searching, 1 Peer Through Depths looks through 10 cards deep if you have an active Ascension. 10 Cards! With so many cantrips in your deck, that'll often be 1/3 of your deck or more! This is why I'll cut Impulse rather than Peer Through Depths to accommodate Force of Will. Your 1 mana cantrips will often find you Pyromancer Ascension fast enough.
Why Legacy Pyromancer Ascension is not Tier 1The short answer is that it is simply too slow. A turn 3 kill in Legacy unhindered is nothing - so many decks can do that! And Legacy Pyromancer Ascension only achieves that if it can draw Gitaxian Probe, another cantrip and Ascension. The odds are bad enough that you might as well play the more consistent Burn.
And I've not touched on how disruption can wreck you. Thoughtseize, Hymn to Tourach, Spell Snare, Qasali Pridemage, Ethersworn Canonist, Force of Will, blah blah blah... Even Stifle gives your opponent 1 more turn before you get Ascension active. I hope you get the drift.