I have never really seen the AI build too many units to kill except on Deity. Usually if they have a ton of units they are so outdated that they are just xp farms for your own units. It becomes especially amusing when you get to logistics Stealth Bombers and just start slaughtering every unit you see. On Emperor or even Immortal I usually find that I am out-producing the AI by the Medieval or Renaissance era.
Domination is a mentality. You have to be aggressive from turn 1 to be most effective. Most Civ players, especially newer players, are too passive. I noticed this in my current Succession Game playing with some other forum-goers. They kept wanting to stop and build wonders or long term-oriented buildings without realizing that the game was already won and we just needed to keep building units and attacking while we had our greatest advantage. Optimum Domination play is about building to an advantage (generally getting an advanced unit before the AI gets a hard counter to that unit) and then pressing that advantage until you win.
This was my approach this game - buy/build units whenever possible, race to Iron and then Steel, ally with militaristic states - including taking Warsaw for Dublin. The Iroqois (pledged to protect Warsaw) were unhappy about that and lost their capital for their trouble - but even with (at that early stage) a catapult and three Swordsmen (initially) against an Iroqois force that at the time had only a few archers and one Mohawk, it took from a war dec in the late BCs to 350 AD to capture the city.
Since then I've had everyone except Siam declare war on me (small map) - my war focus set me back technologically and I'm at least a tech or two behind America, who just captured Taghaza with minutemen (I'm currently researching gunpowder, and am 5th out of 6th in the literacy stakes, down from 1st before the first war began).
I've just seen off waves of attackers from the north (Indian knights and a crossbow) and west (a bunch of Mohawks), with the Americans coming up from the south bolstered by their ally Tyre. I so far haven't seen Babylon despite their also being at war with me - Akkad makes a nice target, but it's not a capital.
The thing the AI has done well this game (most likely by accident of CS map placement) is select CS allies well based on their locations. I can't get to India without going through Helsinki and Edinburgh, and can't afford the happiness hit from holding both, forcing a time delay when it's already 1150 AD. The Americans are allied with Tyre and Genoa, which are between me and their so far undetected territory, and result in the same issue; there's also the prospect of Monaco entering the war if the Iroquois ally them again. Being CSes, of course, they default to the maximum available tech level for their units. My army is as large as I can sustain it financially, mostly now focused on the attack on Taghaza, but also with garrisons and melee defenders around Tombouctou and the former Iroquois capital.
This is the sort of thing I'm getting at - obviously I'm outnumbered having three active enemy civs to fight simultaneously. Advancing will cost me time, funds and happiness (all of which I'm short of) capturing city-states before I get to any capitals - the only civ whose territory I can attack without going through/past CSes is Babylon, and that will require taking out Akkad - it also isn't a good idea since Babylon is far too far from my territory to defend while I remain at war with India.
EDIT: Well, turns out the map is heavily on the AI's side in this game - all approaches to American territory are surrounded by mountains, with only a single narrow route to go through. The attack path closest to me has mountains along one side, an inland sea on the other and a one-hex wide path of hills between; going around there's only a similar path between the mountains and the coast, blocked by the hostile city-states Tyre and Genoa. But repeated wars against every other surviving civ (I took Akkad and Babylon; the Siamese finished that civ) still bog me down.
It's a testament to bad AI and/or the Great Wall (which was conveniently located in the Iroquois capital) that I've been so far victorious while being at war simultaneously with four other civs and nearly all the game's city-states, have captured two CSes, recaptured Taghaza (twice) and razed San Francisco, while clawing my way back up to second place technologically from the bottom of the pack. At the same time it does support my point - I've been at war most of the game, have been on the offensive for most of it, clearly have an edge over all my AI opponents - and into the early 20th Century only two of six capitals have so far fallen.
Shouldn't have ignored India, though - they just captured Dublin.