What @Crossfall said.
It’s very important that Bloodsucking gets Surespell. A single hit without it and you will stop the channel. Additionally, as said, 1 point into each Energy Bolt and Earthquake only. Energy Bolt has a knockback effect, and EQ has a knockdown (which if you get good at practicing you can EQ->Sleep very reliably for a simple combo).
Sleep is one of the simplest and stronger CC’s in the game so maxing this is almost a requirement.
Sink the rest of your points into Reflect Shield. No, not because it’s viable for party play, but because enemies with a small knockback effect on their auto attacks will interrupt Bloodsucking even if you have Surespell up. For those mobs you’ll have to try to CC them but if they are immune then you’ll need Reflect Shield to help keep the channeling alive.
Also @raydensorc while that is a nifty effect it is sadly a bug (C3 Thaum FF here) so I wouldn’t advise it for that reason alone. A single circle of Thaum can be great though if OP is finding himself building into a lot of INT or a lot of CON, but, since he’d have Linker C2 already in the build either way, I likely wouldn’t advise it. The link damage between Bloodsucking and Blood Bath can get pretty solid, and the HP gains from Bloodsucking alone would counter the need to shrink an enemy.
Oh, and, be sure to put 1 point into Bone Pointing as it applies the Kurdaitcha attribute for a chance for double damage (Kurdaitcha attribute can also be learned even if the skill level is 0 which makes Bone Pointing a much better delivery of the attribute) xD