im actually terrible at this game, i just spent an hour and a half on the reptile and lost against the xeno
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.