I think the nodes are called Acuity and Power Overload. You can have your spell damage increase every second. It will continue to increase forever if you only use melee or buff skills. Furthermore, the increased damage will persist between areas and even arena maps.
So you can turn on your Arcane Ascendance outside the arena portal and wait 10 minutes. Then you use the arena key and start with 3000% spell damage. It will continue to increase and it will persist even after waves 15, 30, etc…
If you setup mana strike to have a ranged spell component then you can spam the mana strike without losing your Arcane Ascendance and the spell damage increase.
May also want to put a cap of like 5 minutes/300% damage increase. But I would consider the spell damage increase persisting between areas as a bug.
Edit - 1 minute = 300%, enjoy my bad math