Mana is generally permanently reserved when you cast and maintain auras, however there is a particular passive node called Blood Magic that strips your mana and uses your health for spells instead. Now instead of going all the way through the passive tree to Blood Magic when your build may not call for it, there is an easier way through a unique item or a support gem.
So here's how I got the achievement:
1. Obtain the unique armor "The Covenant." It's under the Spidersilk Robe (lvl 49, INT) class of armor and usually pops up on the trade board every now and then.
MAKE SURE THE ARMOR IS A MINIMUM OF THREE SOCKETS WITH TWO BLUES AND ONE GREEN. THEY DO NOT HAVE TO BE LINKED.
2.Collect three auras with two being 35% reservation cost auras and one 25% reservation cost aura. i.e. Arctic Armor, Discipline, and Purity of Lightning.
3.Equip the auras, use them, and DING achievement unlocked.
****More info added by ProjectSix (thanks btw)
However, the blood magic gem won't work as you describe it I don't think. Getting Blood Magic through the Covenant or the keystone on the passive tree and this works. The support gem has a mana multiplier though. It's 245% at level 1 down to 196% at level 20. So, any aura that reserves 50% with a lvl 20 Blood Magic gem is 98% reserved (50 * 1.96) and it should unlock. Getting the lvl 20 blood magic is a bit of an issue, though.
The best method (without The Covenant) could be a 35% reservation aura with a lvl 1 Blood Magic (35 * 2.45 = 85.75) and then a Clarity linked with the BM as well. Clarity is a flat amount reserved. It's 34 flat at lvl 1, so that's 34 * 2.45 = 83.3. So depending on how much health you have left, you'd level Clarity up until it took up an additional 10% of your health when linked to BM so it's above 95% total.
There's a very useful aura calculator where you could figure out which combo works for you to get it above 95%.. just set your blood magic gem level, which auras, the clarity level, etc.
(include your current life total in the calculator as well) https://poe.mikelat.com/
There's also aura reservation nodes on the tree that reduce the amount reserved that would effect normal ones and any with Blood Magic as well.. that won't apply to most characters but some builds do take them. Also, there's Enlighten support gem which is a *less* mana multiplier.. 96% at lvl 2, 92% at lvl 3 I believe.