Eh, I just paste the gene onto some unit with Blood magic, and then I fuse those units with those genes to create a super unit. You need to kill the king though. It is very easy to create a max gene seedlings with max level 20 and all good traits to SACRIFICE away.But then you need Catalyst to fuse it to a regular unit unless you evolve it to a humanoid. Also, Creation gets seedlings too (if you take the right skill node), and is actually better in some ways at summoning them due to her superior potential for magic.
In some way, it is arguably faster than a spirit because a Seedling increase their level by 4 each evolution (with skill) rather than 3 like Spirits. With Intelligent (which is good for a mage character anyway), a fused Seedling get X3 (+200% XP) the amount of XP instead so it is quite quick to get a max level Seedling for Sacrifice