A good standard Dagath builds needs to use all her base abilities, ~a little over 100 strength, and high range and decent duration.
She's debuff Saryn with a directional finisher. With 2, prime an enemy your allies won't immediately kill. Cast 1 (and recast often) - it will spread Doom to everyone within range. Then flatten them with your 4. For all normal uses, basically base strength + doom will result in a full strip and death. Eximus may take 2-3 casts. 3 is there to buff your CD and bring you back when you get unlucky with gating or someone actually gets close enough to land several hits. Which is unlikely given her kit slows and kills most everything.
If you gotta helminth something , do the 3. The 2-1-4 combo is the whole point of her. Remove one of those and you may as well just play.. Rhino.
This is one content island. Have faith that DE will be DE and will abandon it within the next 3 months. Yes, the randomizer is harsh and advantages no lifers like me who've made 3/4 the frames and a third of weapons viable, but when you're done grinding your way to the rewards you want (incarnons and not much else?) you'll never have to play anything like this again unless you choose to play Soulframe.