I am not a master at PE, but at least against the AI of US and UK I still win, so I guess I can give a little tips, correct me if I am wrong...
The tips is to know what you will use more, infantry or vehicle...
If infantry then you should upgrade tank buster quickly or go T3 quickly, Tank Buster good against enemy that mostly use vehicle while T3 good against spam, then take increase squad size for survivability...
If you focus more on vehicle upgrade advance repair is necessary and build Armored car then Marder or go Panther, Armored Car for Infantry while Marder/Panther for the armor, the PG become passive helper (upgrade with MP 44 or "their-first-upgrade-that-I-forget-name is good way)...
When against US it will be good to focus on Anti-Infantry while UK will be good if you focus on Anti-Building...
No matter what, tank buster is a must at late game...
About doctrine, it can go both way: You can choose when you seem will be defeated or you can choose when you can feel it...
Scorched Earth focus on Map Control which mean you need to be very fast at cap and try as good as possible to trap your enemy...
I can't give anymore tips for this as I am yet to master it...
Well, I hope it useful...
Thanks...