Well, if you can start in, say, Ireland in 1066, and become stronger than a combination of any two or three opponents within a hundred years, you're a better player than I am, and I'd be interested to hear how you play that hundred years. Because, for me, that's the interesting part of the game.
I am sure this can be done much faster but this is how I went from Duke of Essex to Emperor of Britannia in about 100 years without embracing a heresy.
Disclaimer: I realize this is specific to the British isles but I use the same general strategy whenever I want to blob.
1. Revoked barony of Tottenham for extra troops and income.
2. Made all single county dukes and counts around me a tributary. (Counts of surrey and Sussex and duke of Kent)
3. Made the two county Duke of Norfolk my tributary. At this point with my tributaries I can already take on any of the bigger Dukes on the isle one on one.
4. Pressed dejure claims for Northampton and Bedford. Northampton first because it is owned by the Duke of Mercia. This weakened him against me and his vassals.
5. Concentrated on gathering claims that allowed me to create the kingdom of England. All pressed claims were inside the kingdom of England. I tend to fabricate but inviting people with claims works just as good and is cheaper. It is important to be strategic with claims. I want to be able to create or ursurp duchy titles that will give further dejure claims. Since you need more than 50% of a duchy to create or usurp I target duchies with an odd number of counties.
In my case my next claim was a fabricated claim for the whole duchy of Wessex. To fabricate claims on duchies your chancellor needs to have at least 15 diplomacy.
I always keep an eye out for when truces are expiring or have expired due to a rulers death. The ten year truce is what slowed me down the most.
9. Once I created the kingdom of England I vassalalized all the remaining dukes in dejure England.
10. The duke of Northumbria owned 5 counties in the kingdom of Scotland including two of a usurpable duchy. Ursurped and pressed for an easy 6 counties. I only needed 3 more to usurp Scotland and get all its dejure claims.
11. Focused on Wales and then Ireland using the same tactics.
Couple random notes.
If an adventurer of your culture and religion declares war for a county near you help him out. After he wins offer him vassalization.
If a heresy manages to win a rebellion you need to be first to declare holy war. If another Catholic beats you it is an oppurtuntiy lost.