Cassandra

Cassandra has great availability as you have her for the entirety of the game and her personal and passive skills are amazing. Depending on how well you can set her up for her passive, she could even be argued into B tier. In anything below Demonic difficulty, she can be a hard carry; however, in Demonic, she has a little rougher time due to her defensive bulk falling behind the inflated enemies' offensive capabilities. 

Hemomancer - There's really nothing wrong with going Ritualist into Hemomancer as her passive will give her mana back to spam Transfusion and its probably her best class. You can go full into play with her massive HP and Speed so she won't get doubled while she will often double. Using the Karambit will allow her to heal up to 4 times in combat based on the enemy bleed stacks and using the Plague Ring will inflict heavy bleed on enemies when they hit you while the Lifebinder Ring tunes that heal potential way up.

Additionally,  I've found great use of the Simulacrum skill in Demonic. Generally, enemies will tend to prioritize them and can relieve some pressure while whittling the enemies' HP down.  The Waveseer dip takes 11 mana off Simulacrum with both mana glyphs while also providing other decent healing and CC'ing capabilities. Soul Tear is a terrible skill imo as it requires too much setup - it might as well be just as dead as Resurrection in Demonic.

Spirit Keeper - Spirit Keeper's passive makes it that when you enter combat, your abilities get stronger. While Resurrection is a worthless skill in Demonic, Spirit Bomb is so good, that it doesn't matter. My suggestion is a Ritualist dip beforehand. You could make the build below potentially a lot more busted with the Plague Ring and some HP Elixirs; however, even Transfusion alone keeps her alive against most threats in the event you use her as an off tank. 

Tidecaller - While there is absolutely nothing wrong with going Waveseer into Tidecaller and (in all honesty) it is a superior build in most cases, I think there is a case to be made with this build with Cassandra. Positioning Cassandra is important because while you lose out on the reduced mana cost, Cassandra gets mana back with every enemy defeated within 2 tiles (for this reason I use Hurricane Ring so she can swap between front and backline support easier). With an Elementalist dip, you synergize both class passives in that using a skill grants you both 4 power and 4 mastery. Like an Elementalist, you want to stay out of combat as long as possible to go in and secure a kill with the accumulated stacks of power (and crit from Destiny Ring).