Hi,
Sentinel Skill Shield Rush
Nodes allocated: Mana Memory and Rush of Sacrifice
This way Shield Rush no longer has a cooldown and should refund all of the mana spent on it. However if you use the skill close to obstacles or sometimes just by randomly spamming it with right click, the mana will not be refunded. It should not be possible to go to negative mana with this.
Easily reproducible by just allocating the two nodes and spamming the skill near the Target Dummy in the End of Time.
Thanks!