Are we assuming sisal is a 3/3 at the start of the puzzle of after Loki’s is cast?

This....doesn't work. Holding priority on the ETB doesn't mean you don't have to choose a target for the ETB when it's put on the stack. You have to choose a scepter target (when the etb is put on the stack) before Selvala is in play and a valid target. There's no point at which you can 1) get the increase in power from scepter 2) activate sisay, have it resolve, and selvala be on the battlefield 3) target selvala with scepter's etb. What am I missing?

Solution that still works, not the author's solution: ||Float CCC RR from Mana Vault and Tinder Wall sac|| ||Cast Loki's for RCC leaving RC in the pool|| ||Loki's Es, targeting Noble Hierarch, holding prio float U from Noble leaving URC in the pool|| ||Untap Noble from Loki's, Sisay in now a 3/3|| ||Generate BG from Bayou and Chrome Mox, cast Tyvar for BGC leaving UR in the pool, Sisay is now a 5/5|| ||Generate WBG from Tunder, Arcane Signet, Noble|| ||Spin Sisay leaving no mana floating, grab Selvala|| ||Generate G from Loki's and activate Selvala generating WUBGG|| ||Activate Tyvar targeting Selvala|| ||Use G and activae Selvala again, generating WUBRG, leaving WWUUBBRGG in pool|| ||Spin Sisay, leaving WUBG floating, grab Derevi|| ||Derevi Es, untap Selvala, Sisay is now a 7/7|| ||Can end in various combos at this point, easiest is Emiel Generate Infinite mana and going into the Mount Doom loops||

@mwaha this is not true. the etb is a triggered ability and can be responded to with another activation

@TurnTables yes the ability can be responded too, but the target is declared when the ability is put onto the stack. Since Selvala is not on the field at the time of thetriggered ability it will not be a legal target. 115.1d A triggered ability is targeted if it identifies something it will affect by using the phrase “target [something],” where the “something” is a phrase that describes an object and/or player. The target(s) are chosen as the ability is put on the stack; see rule 603.3d.

1.) Cast Tyvar with Chrome Mox and Mana Vault, float 2 2.) Use Tinderwall and floating mana to cast loki's scepter 3.) Use red floating, noble hierarch, Loki's scepter, Archane Signet, Tundra to activate sisay now that she is a 5/5, Grab Selvala 4.) Use Bayu to activate selvala, netting 5 (WUBRG) 5.) Grab loreth. Tap Loreth untap Septer 6.) use Tyvar to untap Selvala 7.) tap Loki's Scepter to activate Selvala, netting 6 mana (Make sure there is an extra green) 8.) Activate Sisay to grab Marvin, Murderous Mimic 9.) activate Marvin Mimic to untap Loreth and selvala, presenting Infinite 10.) Use Sisay to grab Emiel 11.) Use Sisay to grab Mount Doom 12.) Use Emiel to flicker Mount Doom