For big damage, there’s not much of a secret to discuss. It’s how the Shaman class is designed to function.
Shamans are designed to start weak and build up damage potential over time. This is achieved by stacking Jujus on the player, which accumulate over time as a run progresses. Currently, in 0.37, there is no cap on the stackable limit of Jujus. There’s a point where the damage gets to be so high that enemies just die, which serves a form of a damage limit. But, if you can get there, it is a mostly irrelevant limit. As event dungeons have the longest runs currently, Shamans have the most time to snowball stackable Jujus by the final boss, which can result in damage numbers such as what is posted in the screenshot.
Of course, as with any update, if there is the re-balancing expected in 0.38 with spells and items, some mechanics may change. Relic Ring stacking to speed up Ultimate generation and Juju potion creation could very well receive the nerfbat in 0.38.
That said, there are multiple ways to pile up the damage on a given turn. This can be said for any class and not just the Shaman, look for gear that provides secondary effects for matching the desired color of the class. In the case of the Shaman, that is Green. So look for gear that provides secondary effects when Green is matched. For example, Juju potions count as Green matches (that’s a trigger)… which spawns more green gems that could match again on the same turn (that’s another trigger potential), which could skyfall more green… and so on. (Side note, Assassins can do the exact same thing with their Ultimate and Purple gems.)
There are posts about players generating too much Speed on these forums. But, what about other, more beneficial stats such as Power, Crit Chance, and so on? There’s plenty of gear out there that can do this now (and likely more once gear effects are rebalanced fully). Loop those stats up and you can start doing some big damage by the end of each run.