I just love the Reaper summon. When I saw you could summon the Angel of Death, I just knew this would be awesome, and it is. He hits for absolutely massive damage, heals himself in the process, doesn't cost much and has a short cooldown. Easily the best counter to Sun Kings and every other very tough enemy, though they can destroy him quickly, respawn him right back and keep slaughtering.
In Survival mode I was able to stall the enemies indefinitely on the very last platform. Since there is barely any room to escape, the Reaper tears them apart there. Demolish whatever progress they make with an upgraded Earthquake. I also had an unupgraded Polymorph, that was very useful to take out the big soldiers whenever the Reaper couldn't handle them, despite being unupgraded. I ended up letting the enemies win after 40 minutes of this, because as long as I kept going they couldn't do ANYTHING against me.