If your opponent controls a face-up monster(s) that was Special Summoned from the Extra Deck and you do not, while this card is in your hand or Graveyard: You can Special Summon this card, but banish it when it leaves the field. You can only use this effect of "Mirror Resonator" once per turn. Once per turn: You can target 1 face-up monster your opponent controls; treat this card's Level as that monster's original Level if this card is used for a Synchro Summon this turn.
Can Special Summon itself from the hand or graveyard without cost.
Can manipulate own level.
Effects depend on what monsters your opponent controls.
Is banished when leaving the field after being Special Summoned by own effect.
You can use Mirror Resonator as a tribute for summonings and effect costs.
Mirror Resonator can be used as tribute when summoning the Monarchs. Monarch decks don’t really have any Extra Deck capabilities, so Mirror Resonator will be able to fulfill his Special Summoning condition.
Copy the level of a level-8 or higher monster so that you can use Advance Draw on Mirror Resonator and draw 2 cards. Copy the level of a level-5 or higher monster so that you can use Mirror Resonator as the sole tribute for a level-7 or higher monster with Advance Force on the field.
Being able to adjust his own level gives him some flexibility as a Synchro material.
It's a potentially free summon that can also come from the graveyard, and it allows you to change it's level. This is an incredibly good generic tuner because of that, and when XYZ comes out, that's even more you can play around with
A good idea. Thankfully this guy doesn't have annoying restrictions like "you can only synchro summon the turn you activate this effect".
First effect is just to get your shenanigans going.
Second effect is for the level change effect to synchro against certain decks. Like some decks use a good number of lv4 monsters, etc. Possibly XYZ as well.