I am usually playing on 7th difficulty, and usually the AI looks very intimidating with their initial attack, but quickly (10-15 turns) retreats if you can put some basic defense around the city they are attacking.
Well, defending is one thing, but aggression with weaker units can often be just impossible. It depends on terrain, number of units, if you have any UU, and if the target is a militaristic civ. If the target is Tradition on flat lands without too many rivers, it may be an easy target even with a tech disadvantage. If it is Authority Sweden then it is probably impossible, unless you strike them first while they have a very weak army. I was actually able to snipe Sweden out of the game with early Swordsmen attack in 2 of my last 3 games, while in the last game I was fighting them in the Tercio tech (we were tech equal) and it was an absolute nightmare.
1) One thing I learned relatively recently is how powerful roads (and workers overall) are during a war. Before this, for some reason I was always hesitant to actively use workes on the frontlines, I thought of them as purely civil units which connect cities and build improvements. Yes, they consume gold (both workers and roads), but you can save and heal a lot of units if you spam roads and don't push into roadless territories. Play defensively around your farthest road line and build them as you slowly push forward. This works better in the early-game as city borders aren't as huge yet and you usually can build roads up to 2nd city radius. You want to defeat enemy's army in yours/neutral territories where you can benefit from your roads, and only then assault cities. Put Melee units in first row on defense, don't attack with them, and use Bows/Skirmishers/Mounted to deal damage. The terrain can be very disadvantageous for Bows, but you can remove forests/jungles if they are in the way. Roads help Mounted units, but pushing the frontline behind rivers can be very hard. Also building forts for your Melee units isn't a bad idea and can help you push into a difficult terrain.
2) The other thing is saving units and killing enemy units (mindblowing, I know). Calculate if the enemy can kill your unit, or if it can be surrounded the next turn without the escape route, and retreat or switch the unit if needed. I usually retreat/switch if a Melee unit is under 50-40 HP and can be attacked 2-3 times (should also count possible Mounted attacks from out of view). Sometimes, if I have no full HP units to switch places, I retreat the whole army, but that can be dangerous if the roads you built are in neutral territory, as the enemy will benefit from them (you can pillage them and restore later). By killing enemy units you force them to spend production on building new ones, which cripples their development. I don't think you can outright win wars against technologically superior enemy, you usually need to go to attrition warfare for some time to catch up with them. Another good thing is to have lots of spare happiness, so that the enemy goes into unhappiness from War Attrition in a long war, which damages their development quite noticeably and gives -10% CS debuff to units.
3) And last but not least: from my recent experience, Woodman and Amphibious promotions sometimes can be a gamechanger. My problem is that I am very greedy and want to get to other top promotions asap, so I almost never take those two, as they are situational and can be useless in many cases. But if the area is full of forests/jungles or rivers/lakes and you are stuck, they can solve the problem.
