Introduction
This is my recommend [Saryn] setup for Defense and Sanctuary Onslaught, though you can comfortably bring it to any mission type.

Miasma and Spores combine to become your bread-and-butter nuke; prime a group with Spores and press 4 to douse them with Viral damage, which often does enough burst damage to wipe out enemies outside of Endurance content. Toxic Lash gives you access to unmergeable Toxin damage, and facilitates your Spore spread. Pair this setup with something like the [Ignis Wraith] for easy Spore spreading.

Molt is your emergency escape button. If you're taking heavy damage, pop your Molt to avoid damage.

Substitutes
You can replace [Cunning Drift] for [Power Drift] for a bit more damage on your Spores and a slightly higher Toxic Lash buff. You can also replace it for any other Exilus mod of your choosing - you won't really notice the range loss except on the biggest Defense/[Sanctuary] Onslaught tiles.