Spectre - ''Tiers'' and what to use

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Blue mobs are +1 to the level
Rare mobs are +2 to the level

No, they drop items of +1/+2 itemlevel, they themselves are the same level as other monsters in the area.
They get a bunch of damage/life/drop/etc bonuses for being rare/magic, but that doesn't matter to Raise Spectre, only Dominating Blow.