@ Mirthadir
True its difficult to get a unit to this point and yes pbviously you're going to support your armies with bombers/choppers/missles and whatnot because there is no such thing as an effective blitzkrieg w/o their aide. I wasn;t suggesting one uses commando units w/o air support. I was simply suggesting that in the event that the commando promo is available one should take it as in the light of effective support it can double or better the city conquest rate if exploited properly. And yes, like any speed conquer strat, its going to cost one some risks in terms of unit loss in order to expoit it.
As far as the pre-mil sci civs one usually has a plethora of high xp units left over after early conquests. Its easy to contrive an army of High xp commandos for later left over from early wars or generated from the GG produced. I wasn;t inferring that one should have praetorian 200xp commandos with lvl 6 combat and city raider 3.
My point is that in order to take a city you need to either:
1. Actually kill everything with nukes/choppers/missiles.
2. Use bombers and leave all units with minimal heath. Then you need normally 4, though sometimes FAR more to administer the final blow.
In the former case you need exactly ONE commando unit. Not an army of them. The devil comes from reinforcing the city with actual defensive forces. You get to airlift in one unit, good. But if it is truly mobile warfare you need far more. Depending on sea access you might be able to dump in choppers (fine unless they have SAM infantry or worse mobile SAMs) and possibly paratroops. In the modern armor age this generally is not sufficient garrison to keep the city when the enemy SoD hits.
In the latter case, you are going to need at least 3 commandos and more like 6. If they aren't tanks or cav they will be sitting ducks outside the city at the end of the turn. Again you have the problem that combat IV anything is not good garrison material for the counter attack.
Regarding old units with high XP, sorry but that doesn't work particularly well. Unless said unit is lead by a GG, all that XP vanishes down to 10 with promotion (in theory you could get your prat up to lvl 5 and save the promotion until you get military science, but this requires you to sacrifice the utility of that promotion for many, many turns). Further most ancient UUs don't take the combat promotion line, seriously how many prats do you have that go combat III as opposed to CR III or Combat II/shock or Combat II/cover? Ancient units that do go combat tend to be mounted and for the purposes of taking cities, they top out at cavalry.
Again for non-chas units that need 5 promos require you to win SO many battles, even for hoards of ancient units the odds of them surviving even just 95% odds every time are nil.
Do you have a save you might be able to show me?