For some promotions you have to battle and battle or you must have West Point,Pentagon and Bunch of GG and change civics.I tweaked XML so that some promotions like Commando,Ace,Medic III need less requirements one level down.

Its obviously entirely up to you as its your game, but I think you'll find if you manage your battles properly, you can acquire the more "expensive" promotions fairly easily with a barracks/stables and the correct civics.

Great generals also help a lot.

If you're still having trouble an aggressive leader (combat 1 to start for some of your units) will also give you a head start.

But as I said, you bought the game, do what you like with it.

Just expect some abuse from some posters for changing the game to make it "easier"

starts abusing

It is a bit depressing after the 17 point mark. Easy enough to get up to there with a carefully managed military production city and a couple of fights - but then the crawl up to 26 is interminable. Especially so as you will tend to have other units in your stack on 15/16 points who you will want to use before you start trying to upgrade the level 5 / 6-ers.

In fact, mathematically, you would probably find an offensive stack of units on 9 points is more 'useful' than a promoted level 6 stack, because they will all get perhaps 2 upgrade-heals in the course of your assault. That might well outweigh the nugatory advantage of (say) 20% extra combat strength - especially against weak opponents.

Well I think there is a reason commando is so high, it is Really valuable, especially in the age of railroads.

I've had untis that have lasted ages and upgrades that have gone past 100 points. These of course are generals, but still, with a charasmatic leader...