The most common tips are; learn the skills of your character and the boss. You need to know what movement predicts what ability.
Otherwise, if you can't do it, try to find someone to carry.
For sidekicks, I use Malice, Myriad, Lote and Wilson Fey.
Enkidra is also really good for Shaggai because his skill damages through shields.
I take Lote for the AoE damage when Shaggai spawns his insects.
If you don't know it yet, you need to remove all insects first, otherwise his shield will stay.
uhm not sure if you are talking about first boss of villa skylla or a different boss, if it's the mantis man you can interrupt his long cast with Hera (hera can stun enemy with strike immunity) and you can get 2x more time to do damage before he shields again.
edit: this also works vs hyperion, when he casts his "certain death sure kill stun of doom" you can counterattack with hera so he doesn't kill you with meteor.
For Shaggai, as mentioned, kill his spiders and then you can attack him. If you're not at a point where you've got mods you want (granted, Iza is pretty solid even without them) it's a good idea to take some Sidekicks that interrupt, like Sarasvati, Yume, Hera, and interrupt his cast a bit before his castbar fills up. It's not absolutely necessary, but the longer the fight drags on, the more webbing will be on the ground, and then you're gonna have a bad time.