Just keep Transcending. Maximise your Cell-growth via all Proteins, Tissues and not more than 7 Neurons.
If you only focus on Transcending, you'll reach 1Q Cells at around +2.000% Growth.
In the beginning, don't go higher than the 1,5k Cell generator and start raising Genes as soon as possible.
Once you have multiple Gene-generators and one or two auto-clickers (Tissue) get into Neurons, but never unlock more Neuron-generators than you have Gene-Generators.
Those allow you the highest Cell-production, which should easily avail you 15 Billion Cells to create the automated Xenocyte killer. Once you have that, go nuts with Cell-generators until you have enough to Transcend.