Summon Shadow Knight on F2, damaging the gate and generating a first boost for Selundar.
Play Onyx Tome, deal two damage to the full life Sniper (two more boosts for your summoner) and retrieve a Shadow Mage.
Summon him on D2.
Return the Sniper on E3 to your hand with "Return to Shadows" and summon a Truth Seeker on its place. Selundar is back only 1 boost now.