There are different approaches to this issue.
I am currently able to win with any civ against any civ in land maps in TOUGH level (but not tougher, still yet to crack it). You can do this by doing multiple different ways. The easiest civilization is Chinese as it has great defensive abilities and instant citizens to speed up economy.
Build order:
SCI1, CIV1 (in the same time, max out your timber +70 and build 5 farms with citizens), build second city, com1, hit production cap on timber/farms, either sci2 or mil1, build barracks and a tower in the forward city, create 1-2 bowman, garrison them into the cities (so it'll automatically fire) if they scout you.
This only works with the computer player (with humans you'd b eraided by now), so what you do is a delay classical. You hit production cap after you get SCI2 and COM2 (no classical = substituting food for knowledge) and CIV2 (third city, bring a couple of bowman up for raid defenses), then either classical or mil2 and then classical, immediately build three "schools" , and start building up metals. Build a stables immediately create a few HA's, then raid them (you hit a copule of citizens, the move them back to the stables to heal, and so on, you need to do this quite a while), build a siege factory, and build a castle.You should be hitting 150 production cap in at least food/wood and should be close to 150 in metal (use production enhance buildings). The three schools you built should give you enough knowledge to advance to medieval almost after you build them (so basically you skip upgrading your military until medieval). Go to medieval upgrade everything and hit them with your army.
Works with computer all the time, works with players that are booming and not attacking you (they'd go classical first but would have to spend a lot of time waiting for knowledge to get up to upgrade everything). But weak against early raids.
If you're having problems with AI< play against the Greek, they're usually the weakest, then other defensive nations then offensive ones.With this you should be able to beat Tough level and able to basically take 1-2 of his cities by enlightenment time. The game may be over before industrial, but i always play unti modern and watch his calvary trying to chrage my machine guns :-)