So far, I haven't been successful in recreating it since the helper doesn't show up at all. I'm quite a noob at this so I'm sure there's at least a bajillion things I did wrong even with the help of a helper creating guide. I'll insert the code I used down below.
AIR file
; AV Mirror Image
[Begin Action 790]
775,0, 0,0, 1
775,1, 0,0, 1
; AV Mirror Image Vanish
[Begin Action 791]
194,0, 0,0, 3
194,1, 0,0, 3
194,2, 0,0, -1
What I inserted into the "Dark Force" code
[State 770, AV Mirror Image]
type = helper
trigger1 = Anim = 770 && AnimElemTime(22) < 0
helpertype = normal
stateno = 790
ID = 790
pos = 50,0
name = "AV Mirror Image"
postype = p1
persistent = 0
keyctrl = 1
Helpers.cns file
;=====================<ASTRAL VISION MIRROR IMAGE>============================
[Statedef 790]
type = S
physics = N
movetype = S
anim = 790
ctrl = 0
velset = 0, 0
sprpriority = 1
[State 790, Fade In]
type = Trans
trigger1 = Anim = 790 && AnimElemTime(7)<0
trans = addalpha
alpha = ceil(5.7 * AnimElemTime(1)), ceil(256 -5.7 * AnimElemTime(1))
[State 790, 2] ;this will allow the helper to replicate the same animation than the root
type = ChangeAnim
triggerall = SelfAnimExist(root,Anim) ;prevent debug error on custom anims
trigger1 = Anim != (root, anim)
trigger2 = AnimElemNo(0) > (root, AnimElemNo(0))
value = root, anim
elem = root,AnimElemNo(0)
ignorehitpause = 1
[State 790, End]
type = ChangeState
trigger1 = Anim=771
value = 791
;AV Mirror Image Fade away
[Statedef 791]
type = A
physics = N
anim = 791
sprpriority = 1
[State 791, No AfterImages]
type = AfterImageTime
trigger1 = !Time
time = 0
ignorehitpause = 1
[State 791, Fade]
type = trans
trigger1 = 1
trans = addalpha
alpha = ceil(256 -8.5 * time), ceil(8.5 * time)
[State 791, Gravity]
type = VelAdd
trigger1 = 1
y = 0.5