1. Mercs are roughly equivalent to 3-4 of their normal counter parts ie jaegers = buffed up skirms. You WILL take heavy losses against mercs, but this doesn't mean you are losing the battle. Hang in there!
2. Black Riders are probably the most used merc unit & for good reason. They counter Curassier/cav, snack on dragoons and have high melee armor. Your best bet for bringing them down are concentrated fire from ranged units as they will just "shoot and scoot" against melee units.
3. Jaegers also utilize the "shoot and scoot" bonus from careful micro so cav isn't particularly effective against them unless you can hit them from two sides. I've found xbows to be cheap 3rd age counters for bringing them down.
4. This "should" be common knowledge, but NEVER EVER fight mercs w/ a unit that the mercs get a bonus against ie swiss pikemen vs uhlans.
5. Since mercs have such high Hp a good player will watch them carefully & pull wounded mercs out of battle to be healed. Concentrate your fire on ONE merc at a time so you ensure that you will KILL it. This way your opponent doesn't have a chance to heal them.
6. I find it preferable to throw large amounts of "junk" troops at mercs rather than smaller amounts of elite units. This is because mercs are as "elite" as units get and so in a 1v1 fight will always beat your top units (provided your unit doesn't have a significant bonus vs them). Masses of cheap troops can swarm mercs and concentrate ALOT of focus fire on individual mercs. Since mercs can only attack 1 unit at a time most of your units are free to FF them to death.
7. Ok, you know your opponent likes using mercs so you raid his gold pits, but he STILL gets his mercs out, how? He's shipping gold from the hc. If someone is going for mercs, chances are they will get at least 1 batch of them out. Don't be surprised, just expect it.
8. Rushing someone who is going for mercs isn't a great idea. A true rushing economy is very lean so if your enemy gets his mercs out you may not have enough resources to deal w/ the threat + his mercs WILL take down your tiny forward base leaving you w/ out the ability to produce units until you rebuild them.
9. Mercs take up ALOT of population. Use this to your advantage. While your larger force is trying to combat his mercs, RAID him! Picking off just a few villagers every couple of minutes can win you the game.
10. Some players hit and run w/ their mercs and will frustrate you greatly. Walling off choke points early can help negate this threat while towers allow you a quick garrison option and enough time to get your quicker units over to help.
11. Skirmishers are your friend. These guys are merc killing machines. They can "shoot n scoot" and their focus fire is lethal even against artillery. They are probably your best anti merc option when correctly micro'd.
12. My last point is map control. If someone is specifically going for "fast mercs" then their econ is usually very small and clustered closely around their starting tc. Use this to your advantage and try and secure key resource locations w/ towers/walls/troops/buildings.
I hope this helps some of the players who have been having trouble against merc intensive strategies.
-Zifnab