you could probably test this pretty easily. find a stun immune mob, cast a few stuns on it, then see how easy it is for someone else to steal agro.
not sure if level or stun duration effects it, but you could test that pretty easily too i suspect. devs of this game tend to prefer people figuring out how things work via testing if possible, from what i've seen.
I did some minor testing to help clear up some of the questions related to stun/blind/flame of light/bash agro. I used my pally and shaman.
Relevant stats below:
Shaman - aggression -2, all AA, no wrist proc, no reactive procs, -6% spec bonus
Paladin w/o shield - aggression +7, all AA, no wrist proc, no reactive procs, 20% spec bonus
Paladin with shield - aggression +9, all AA, no wrist proc, no reactive procs, anger proc on bash, 20% spec bonus
Test format:
I would cast either Turgur's Insects or Cripple and let the mob run into melee range of the shaman. I would keep the paladin out of melee range, and then cast said spells. (Unless I was testing bash, then I would run into melee range as soon as the mob was in melee range of the shaman.)
1 cast vs. how many casts it took to get agro.
*I did each iteration below at least 5 times (some were done more like 20 times...) (Only a few runs were done with AE spells.)
Rust - Immune to stun - near zone in area (light and dark blue)
--------Stuns
1 turgur's/cripple - 2 Word of the Crusader (AE)
1 turgur's/cripple - 1-2 Words of Thunder (light blue mobs 1 cast, db mobs 2 casts)
1 turgur's/cripple - 2 Words of Tranquility
1 turgur's/cripple - 2 Force
1 turgur's/cripple - 2 Holy Might
1 turgur's/cripple - 2 Stun
1 turgur's/cripple - 2 Desist
1 turgur's/cripple - 2 Cease
--------Blinds
1 turgur's/cripple - 2 Wave of Light (AE)
1 turgur's/cripple - 2 Blinding Light
1 turgur's/cripple - 2 Gleam of light (AE)
1 turgur's/cripple - 2 flash of Light
--------DD
1 turgur's/cripple - 1-2 flame of light (light blue mobs 1 cast, db mobs 2 casts)
--------Mix
1 turgur's - 1 words of tranquility + 1 cease (if cease resists it takes another cast that lands)
1 turgur's - 1 Words of tranquility + 1 flash of light
--------Bash (same result types with both cripple and turgurs)
1 turgur's - no shield: 17, miss 16, 16, 9, 15, miss, miss, miss, 57, 13, 22, 13, 15, 14... (dark blue mob - similar to all tests run with no shield)
1 turgur's - shield hits: 11, miss, 20, 13 (dark blue mob)
1 turgur's - shield hits: miss, 17, 22 (dark blue mob)
1 turgur's - shield hits: miss, miss, miss, miss, miss, 13 (light blue mob)
1 turgur's - Shield hits: 14 (light blue)
1 turgur's - Shield hits: 16, 14, 13 (dark blue mob + anger proc)
1 turgur's - Shield hits: 15 (light blue mob)
Sundered mountains - stunable - vulfwere pup huts (ultra green)
--------Blinds
1 turgur's/cripple - 15 Blinding light
--------Stuns
1 turgur's/cripple - 5 cease/5 desist/4 stun (14 total stuns)
Conclusions:
1. From testing it is 100% clear that blind has to run/overwrite before you get agro for a second blind cast. (There maybe some minor bit of agro from chain casting the same blind, but it is almost nothing. This is consistent with the way cripple works too.) This is not new info...
2. It became obvious that a resisted stun cast gives almost no agro (not to be confused with stun immune). Again, not really new information.
3. I did some testing at Eldenals Mansion zone in, where mobs can be stunned, and I believe that it does not matter if a mobs is immune to stun or not. The agro amount is relatively the same as long as the spell lands.
4. I think it is obvious that a shield is required for 'bash agro' to work. This kind of sucks since it is nice to use a 2h weapon during xp runs.
5. The agro with ultra green mobs... wtf...
6. It looks like for the most part, each stun gives similar agro with the exception of Words of Thunder being significantly more agro.
7. Flame of Light is worth casting because it gives relatively the same agro as the other agro spells (words of thunder aside) and it is unresistable.
8. I did not test how proximity to the mob affects agro.