Do Mother North. There are a ton of snipers and other heroes out there that can substitute Lianna as damage dealer, even from the ranks of 4 star greens. But there are only 2 revivers in this game in the current meta and that would be Mother North and Alberich. These 2 heroes are the real game changers as you may have a chance to be able to defeat a much tougher enemy both in wars and in raids due to their revive skills. With Mother North, not only she provides a chance to revive fallen allies, she also heals and summons minions, making your whole team a bit more challenging to overcome, giving you a higher chance of victory when attacking or defending. She offers a lot to the whole team than Lianna, who only deals damage but cannot even one-shot a maxed and unscathed enemy hero of equal and decent caliber even when Lianna is fully emblemed, even if Lianna is regarded as the best sniper due to her raw damage output to a single target.
EDIT: As an addition for consideration, you can always get Lianna at any portal that can summon a hero, even from the Training Camps. But MN and Albe are heroes that can only be summoned very, very, very rarely. Albe, if featured in Atlantis, and MN, during Christmas events. That insinuates how good those two are.