Okay, I am a fairly new FE gamer (have beaten all of the FE games I've played, but I don't consider myself an expert), so this may sound nooby or even dumb to some who are more experienced.
Most cheap things have been considered. The only thing I'd consider cheap would be a team of sages with fored Thorons,RNG abuse and Umbra (my personal favorite card, as it promotes ambush over attack, something I pracatice maybe a little too willingly...). The main reasons I consider this team cheap is because the only way you can get an effective mage killer is either by RNG abuse yourself, or hacking (effective mage killers having high movement range, strength, and resistance). Umbra also adds another element, virtually eliminating ranged play. While that may seem bad for the sages, on my relatively nooby main online team, my sage (Lena, due to convienience) has scored 42+ on most enemies. That's a one turn or even one hit kill for most. Now imagine there where 5 of them that could do that... A modification to that would be a Falco or Draco with Iotes Shield and RNG abuse for high def. as a scout.
This is just a personal opinion due to some very annoying matches I've played where Lena literally was my only ace in the hole, allowing me to win, as how this game skimps on resistance for non Curates/Bishops (Lena start as Bishop to max resistance, then Sage to max mag works well for me).