This is a guy who started building a worker on Boer Deity I wouldn't ask him for advice
How I dealt with Portugal is grovel at their feet until you take out England and Germany. If they DoW you earlier than you are ready then reload as they will 300 vs jerksies you, sure you'll kill a million but there will be 50 million waiting to swarm in and eventually they will find a crack, then there's the cannons... Once you have those coastal cities and a nice navy of Privateers and captured English and German ships you can manhandle them.
Another reason you don't want to DoW Portugal is that they are the only real threat from the south to Ethiopia and really their only direct threat. Italy hates them but has to go through Ottomans AND Egypt to start whacking at them which takes about 60 turns. So you don't want Portugal sending all it's troops your way and leaving Ethiopia to run wild.
There is another enemy to the west of Ethiopia that you may never meet which also hate them but he is very weak I can't even remember his name.
I in my game I did take 4 Portugal cities but I it was more for fun than points, you don't get much from con-questing to far out, keep rotating and healing your troops.
This would be a really good scenario achievement if it was a close game every time, but it's not. Most every game you will have 1 guy up north beat everyone else by 2-3k that is simply poor deity design.