Currently the challenges at Eidolon are:
Shattering Rain: can you dodge a death zone (super easy)
Geysers: can you dodge a death zone (hard).
Yellow Orbs: can you kill the orbs before their summons cause problems.
Green Orbs: can you kill the orbs before they blow up.
Red Orbs: can you cc the orbs. Can you kill the orbs before the DoT they create becomes unmanageable.
Suggestion for how to create a greater diversity of challenge:
Yellow Orbs: replace with unattackable portals which summon respawning mobs for a limited duration (this turns it into a tanking problem). I suggest the portal fx from NemCon.
Green Orbs: no change.
Red Orbs: change siphon nightmares from variable (magnitude 1-9 based on number and count of cced orbs) to fixed (say, around magnitude 5; or make it resistible to give tough dps a reason to exist). Phase ends when all orbs defeated, instead of fixed duration. This adds a significant healing challenge and downrates the dps requirements.
Some cosmetics also:
I would make red orbs generate the same screen-altering vfx that the similar phase in NemCon generates.
I would make red orbs spawn at 50-100' away, then move towards him and only heal him when in range (using the healing fx from NemCon).