Sideboard Plan(s)

Against Aggro:
-1 Approach of the Second Sun
-1 Glimmer of Genius
-1 Profane Procession
-1 Disallow
-2 Search for Azcanta

+1 Scarab God
+2 Regal Caracal
+1 Settle the Wreckage
+2 Legion's Landing

This is basically thinning out the top end of your curve for lower curve interaction and life gain. Legion's Landing gives you some lifelink or at least a blocker if you don't have turn one removal. Regal Caracal helps stabilize mid game with lifelink as well. The Scarab God plus a flipped Arguel's Blood Fast makes for consistent lifegain by chump blocking with him and then sacrificing him.

That said, you keep in Arguel's Blood Fast with very little intent of activating the ability - you can expect your opponent to hit you down that low. Sometimes activate it once or twice when looking to flip it at six or seven life. Or two find removal.

Against Control:

-1 Approach of the Second Sun
-2 Fumigate
-2 Settle the Wreckage
-1 Supreme Will
-2 Essence Scatter
-2 Moment of Craving
-4 Fatal Push

+1 Scarab God
+2 Regal Caracal
+1 Doomfall
+3 Glint-Sleeve Siphoner
+3 Kitesail Freebooter
+2 Negate
+2 Legion's Landing

Most control sideboards go heavy on hate for Approach of the Second Sun with Duress, Negate, and Lost Legacy, so the goal is to make that sideboard plan less useful by turning into a midrange shell.

Kitesail Freebooter, Glint-Sleeve Siphoner, and Legion's Landing make a mass of the cards brought in to pressure the field and may potentially flip Legion's Landing, which is near impossible for most control decks without removal to come back from.

Going to game three if at all, it's a all a mind trick. Pickup your sideboard as if you're sideboarding regardless, but consider going back to full Approach as they may be putting in removal against you and that means dead cards in their hand if you take out the creatures. It is hard to gauge how to sideboard back in as it depends on what tools your opponent has shown and if they are worried about the low curve threats at all.

Against Midrange:

Still testing...