Stupid Praxis

Sauropod Wrangler, Trail Maker, and Seasoned Spelunker help ramp, and double their efficacy with Mysterium Orb.

Seasoned Spelunker doubles as an activator for himself (he turns into a reasonably big body) and Monolith Guardian. <Insert complaints about Monolith Guardian being a 1/1 for 4 from people who don't include enough relics for consistency.> Mysterium Orb also doubles as an activator for these two units, and will ramp you to 4 for Monolith Guardian a turn before the card can be played.

Waker of Ancients essentially gives you draw worth all non-Heart sentinels in the deck, and gives you a decent clue as to whether you should bottom something with a crest later game. I left the power warp radiant off the sideboard, but she's a good option to use along with Waker in a sentinels deck provided you're using a more scout-heavy deck.

All the explorers give you access to further sentinels during Stirring Sand; with an Orb out, they generally pay for their own activation.

Mysterium Orb's final function in the deck is to give your sentinels Overwhelm and Endurance. You might be able to get away with only 3 copies, but this piece enables all the others so well that you really don't want to miss drawing it if possible.

The new Sandglass Juggernaut is seriously enabled by Mysterium Orb, in that it's nice to have an invulnerable blocker + attacker every turn. Orb also grants Juggernaut Overwhelm so it avoids getting bounced, and Silences will only limit rather than prevent damage from your attacks.

GKT is here mostly for the auto-play value, as self-playing Endurance / Overwhelm giant creatures are pretty okay.

Scourstone Sentinel is lol@draftcard until you chain bond a few of them in on a Monolith Guardian (if Mysterium Orb is up) or Titan (if not). Then they help you get around decks that might otherwise go too wide for you to manage defending against. (Scourstone is an "offensive" defense most of the time.)

Torch, Heart's ping, and Purify are all reasonable removal for small units, as is Sentry for flliers. (Sentry, incidentally, is included as most fliers are low health, it will almost always be activated, and Titan is often not enough in terms of aerial defense--fliers are Big Praxis's big weakness.) Scorpion Wasp also acts as removal for units of arbitrary size, especially fliers.

Tomb Protector will remain in sideboard until the Madness craze fades.