Mage/Dark Wizard Sustain build?

Upon contemplating the Majin and Tasim fights, I've come to the conclusion that, aggravatingly, my reliable build of Morph + Cascade does not work. It is not strong enough. It was strong enough for Zhiraok, Zhiraok, and Gikyo, but not for these two. Which is very annoying.

My playstyle with my Lightbender is to run around mainattacking with Ophanim, trying to dodge with Teleport, and, when I remember, shooting off an Ice Snake. I usually spam Heal to heal faster because damage sometimes hits really, really hard. I'd prefer Mana Regen over Mana Steal. I have 16 stx in total but I'll allot three for a new Fruma Murder Build; what would that build be?

 

This probably won’t be three stacks or less, but here’s a Lament build I made.
https://wynnbuilder-beta.github.io/builder/#CT00+00HOl3yQWjH6VGhJOFohcKSfVUwEkcx+0
Heal spell heals 183% of your max health with perfect rolls which should make damage become barely an inconvenience.

 

Had a looksee. Seems, like a build. ATree looks, buggy. I will probably ignore it. I want my sun.

 

I don’t really like the ATree either, but the nerds who make actually good builds usually ignore the ultimates so I did the same.

 

Well I'm making the alteration to have an ultimate. Even if it reduces my damage. I like ulting.

 

I bought all the items for this build and used it against Tasim. It helped somewhat, but he was still inordinately long and I couldn't get him below four million. However, I made the following substitutions and compromises:

• I chickened out of applying that three stacks I assigned to the build and only spent one. I can't judge the quality of my items, since I don't have Wynntils or any experience in this matter, but I know most of their rolls are half or less.
• The bracelet was very expensive on the Trade Market due to being a quest reward from the quest I am trying to use it to beat. I substituted it with a Unique bracelet from Relos' accessory merchant which seemed to have similar effects.
• Stardew was substantially expensive, so I repurposed a middling-roll Dawnbreak I happened to have in my bank from a chest mythic.
• I did not respec my atree from full Lightbender.
• I did not apply the powders to my Lament, as three T7 powders is ~18 LE worth and I only have four. I don't know how good my Lament is, so I didn't want to risk it.

In consequence, I have not done well.

I made these changes under the knowledge that something was probably going to break and I wouldn't be able to tell. But I don't know what the problem was. Substituting Stardew is the most obvious culprit; low rolls is another suspect, but a much harder one to prosecute because, as mentioned, I don't have Wynntils nor do I know how to judge the quality of an item as it relates to a build I don't even really understand. Those few Riftwalker abilities might have been secretly been critical to the build, and perhaps the powders would have added some extra DPS on top, putting this in a reasonable range. And maybe replacing Shattershard with Wristviewer somehow shattered the whole thing.

With the knowledge that I will cavalierly break things if I don't know what they do - what is actually going on in this build? Which items are important for what? What rolls do they need? Which are expensive but ultimately auxiliary and easy to replace? And: what playstyle am I supposed to use it with? I've been fighting Tasim by trying to stay on one end of the arena, slowly hitting him with Ice Snakes and (when he's nearby) Ophanim strikes while healing from undodged attacks and teleporting through him to the other side when he gets close enough to slaughter me in melee - is that impeding my Tasim-killing effectiveness significantly?