This is my take on the Mardu "ETB shutdown" deck that features Strict Proctor, Hushbringer, and undercosted creatures with ETB downsides (Kroxa, Titan of Death's Hunger, Hunted Nightmare, and Clackbridge Troll).

Hushbringer and Proctor are both capable of turning off the downsides on the three payoff creatures above (Proctor lets you simply decline to pay the tax on the ETB triggers). Both Hushbringer and Proctor also have the potential upside of heavily messing with your opponents' gameplans:

• Witherbloom relies heavily on death triggers;
• Yorion, Rogues, and Angels rely heavily on ETB triggers (there's a real joy to taxing Ruin Crab's landfall triggers with Proctor);
• Only VERY savvy RDW players will notice Strict Proctor's 2-mana tax on Embercleave's attachment trigger.

Proctor + Kroxa is a particularly potent combo, as you can pay for Kroxa's discard effect if you want it but decline the sacrifice effect. And even with the ETB downsides, all three ETB blockees are solid on their own. Hunted Nightmare can be unusually strong depending on the matchup, as Yorion decks in particular will often have no creatures onboard on turn 3, and is a great midgame beater, blocker, or post-Blood on the Snow resurrectee.

Cheap removal spells like Bonecrusher Giant and Heartless Act are all over the place right now, and being able to keep up an ETB-blocker can often determine whether the game is a blowout in your favor or your opponent's, so this deck features 3x Selfless Savior to protect your ETB-blockers and 2x Lurrus of the Dream-Den/2x Blood on the Snow to bring them back. This deck relies more on value and card quality than tempo, and the mana base is pretty tricky; don't be afraid to T1 tapland, T2 Selfless Savior + tapland, T3 Hushbringer/Proctor + tapland. Rushing to play your critical cards will often just eat a removal spell and leave you struggling to find your footing.

This deck also runs a single Drannith Magistrate to have as an available option against Sultai Ultimatum, Mizzix's Mastery, and Adventure/Foretell cards; Elspeth's Nightmare for versatile removal/discard/graveyard hate; 2x Showdown of the Skalds for card draw; and 3x Reidane, God of the Worthy for general utility. Valkmira, Protector's Shield will usually be the more useful side by taxing your opponent's removal effects and bolstering your toughness, but feel free to use your best judgment.