0.) Forget everything regarding the forgotten god and the void. Lesser ranked beings from dormant and weak ascended are burdened by this knowledge. (Most fail here)

1.) Using the Spell, beat the first nightmare and aim for a transcendent or higher ranked aspect. Divine is ideal. The higher ranked aspects future proofs your run.

2.) Surround yourself with trustworthy and powerfully adaptable people. Survive the Solstice and beat the 2nd and 3rd nightmare. Be sure to collect many useful Memories and echos. Try to manifest a true name and aspect legacy as within this time, preferably ASAP. 

3.) Upon reaching sainthood, dedicate your time to cultivate your Will by staying in the Hollow mountains and honing your abilities by fighting great nightmare creatures of higher class like great devils and above. Recognize your source element and try to grow your nascent domain by having a following of people. This is where the aspect ranks, legacy, and true names make or break the run. 

4.) Challenging the 4th nightmare is actually redundant, since if you recognize your source element and can grow your nascent domain, you can just attempt natural supremacy by putting yourself in increasingly dire situations for acts of defiance. 

5.) When reaching supremacy, take over everything and bring everyone to your domain. Your Will should be unbreakable. Conquer the ToA and cultivate apotheosis. Be wary of future and past dangers. By this point, you can attempt to either consume your own domain or do it the natural way. 

This should only take you about 7 real years at the slowest. Maybe less if you entered the real ToA earlier as a saint with your nascent domain and then it would be around 5-6 years in real time. The time you may spend in the ToA would probably be hundreds of thousands of years.

How to speedrun godhood in Shadow Slave