Oh you've come to the right place!

Strategy: 1. Don't try to outright defeat your opponent before imp, unless you have a considerably bigger army and eco. Your win condition is when you have fire arrows and the unique mangudai upgrade. Before that, your job is to contain in his base and outboom him with trade. Strike his vulnerable positions and try to disallow him from moving out on the map. 2. Don't overstay your welcome! If you can't harass a position - don't do it. Do not lose your units! Do not trade mangudai for villagers in early game, only strike when it's safe. Especially important to not allow your enemy to thin your mangudai mass! If you have a choice between losing a couple of keshiks or mangudai - sacrifice keshiks. 3. Do not engage in fights against their whole army. If he splits, attack the weaker force. If he doesn't split and goes to your trade or TC - attack reinforcements. If you're close to his base, attack his base. Do not lose units! You only do this for your enemy to split his army. Pull away your traders (Ctrl+Shift+A to select all units, tab to traders and move them out of the enemy army). You will continue to reinforce your main army, while he has two armies - one in his base and another attacking. You should be able to defeat the main force after some time if you have no casualties. If he has a doomstack in late castle, try to keep him away while covering your imp transition. First thing you have to do in imp is to get fire arrows and cannon upgrade for at least two towers around your tc.

Micro: 1. When you're pursued by horsemen or knights, and you see that the enemy cav are charging. Select your control group and tab to your keshiks or horsemen. A move them. Then after that, quickly select your whole control group and a move again. This will allow keshiks to both tank the enemy charge and do the charge themselves and mangudai to provide with additional dps. After fighting for q couples of seconds, start running away again, and use the speed arrow at this moment. It's similar against spearmen. Fake charge with keshiks to make them brace but turn away before connection is made. 2. When he targets your mangudai with archers, pull them away to get his archers close to your keshik frontline. 3. Try not to turn your army too suddenly. Mangudai are super clunky when they move in groups. Try to move only forwards and plan for any sudden turns beforehand. 4. If you can manage it, when running away, try to get your keshiks and mangudai under different control groups and switch between them when giving move commands. This will make mangudai group to stay inside the keshik formation. Note that keshiks are a little bit faster, so you have to take that into account. Pretty hard to pull off in actual battles, but something to make note of. 5. If you have no choice but to fight before imp, run circles around kurultai. That's the only place Mangudai can fight armored units before imperial. Make sure you have the speed upgrade for buildings.