How can I get a conquest victory with out first winning by domination? Can this be done with out playing a custom game?

Yep. Play a Terra map, and kill all the opponents before they set food on the big new world.

Or, in a "normal" continents/pangaea world, amass a huge force and take out every civ razing every city. If you raze everything you encounter, you won't achieve the "land" percentage.

You don't even have to raze every city you take. Raze the bad ones and keep the useful ones. Some will have Wonders you want or be good to add to the war machine.

Don't build up your culture in your cities. That will stop their cultural boandaries from growing and taking up more land.

The key is to get them all as quickly as possible.

razing is easier. have a look at the winning conditions. if domination comes too near just raze every city you conquer. you won't need any wonder or shrine by then anyways.

Just don't conquer enough of the world to get Domination. When you get to about 50%, start razing everything instead of conquering them. If you're on continents, keep one city on each with an airport and station some troops to dealwith any rogue AI settlers that show up and have your workers railroad everywhere so you can just jump over and deal with anyone who lands on the newly deserted continent. Then just blow through everyone.

It's a pain in the ass to get, especially when you have to consciously avoid winning the game in order to get it, but Conquest has hands-down the best victory movie in the game and it's worth the trouble to watch it.