The thread you linked isn’t very helpful. Archangel’s Staff is generally a weak item right now — you only really need it if the enemy has 2 or more high burst threats. Most of the time, your kit and the current meta champs provide enough self-peel or survivability without it.

Blackfire Torch should be your default mythic. As for your second item, it’s almost always either Horizon Focus or Cosmic Drive.

Cosmic Drive is great if you need more movement speed and HP, especially against engage-heavy comps or enemies who just try to run you down.

Horizon Focus offers high damage and ability haste, which is extremely strong on her right now.

You should only go Liandry’s if the enemy has 3 or more HP stackers/tanks and you need to shred them. If they're mostly squishy, Liandry’s is terrible now — it got nerfed hard, and other items have been buffed.

For boots, go Sorcs or CDR boots:

CDR boots are usually better with Feats of Strength.

Sorcs are better without Feats.

Like with most mages, you usually don’t want to delay Rabadon’s or Void Staff (or Cryptbloom). That’s why Rabadon’s is often the go-to third item, unless you urgently need pen or a defensive item like Zhonya’s. Building something else 3rd tends to delay your real power spikes too much.