well, One thing i'd suggest is setting your light healer up as a healer (currently he's set ups as... I don't even know? what that prof setup is meant to be? Why 3hp? Why does he have no Int prof points? His healing depends on his damage output?? Lv75 light healers are almost always 3int/3dex/full pyro, for the highest attack output, accuracy, and crit chance)

Other than that, usually it's "attack Elaine until she's about to do her 100 mana move, then..." Well, for me it's always been "taunt her to earth tank, thorncoat, then ignore her for three turns to attack her helpers" or "set up ice sphere with water tank and ignore her for three turns to attack her helpers" but uhhhhh,,, obviously THAT doesn't work with a shadow tank
Shadow does get a Taunt move at 65, so my best guess is "hit her with hateful strike, then block with shadow pet for three turns while the other two pets attack her helpers"?
And when she's at about 1/3 hp decimate to take out a chunk of her hp (since your decimate isn't ranked you'll have to lean to her having lower hp when using it)

I've been always told to kill her Helpers first until only Eleane is left. Do I not do that, and just go for Eleane half of the time? Is there also a specific moment where I use Attack 5?

Honestly, I'm just saying the way I do it with my main+secondary teams.
I've got no idea when one might use move 5 on her because I've never done it, since I've never needed to (back when we might possibly have needed it, the cap was lv65 so we wouldn't have been able to use it anyhow.)

You sure could go for her helpers first, I guess? I never bother, since if I take Elaine out first then I don't have to set up defenses again for the rest of the fight (since her healers basically don't do much of anything damage-wise)... but my pets have been Ranked for a few years now, so I have no issues completely overriding any healing that the helpers do to Elaine and finishing all three off in like, 14 turns. My memory of how to fight her with unranked pets is pretty faint since that was... 3 years ago now

Maybe take out one healer, then Elaine, then the other? I don't know.