Mercenary tactics since 1.8.

[WKIII]

You have to research the Administrative and Innovative ideas to lower the mercenary prices and maintenance by nearly 2 thirds.
You also get many more mercenaries once you have researched these ideas, you can get an army of 30k mercenaries in the 1500's if you have a decent income or trade power so you can afford respawning many mercs in a short time, otherwise this won't work at all.

An army of only mercenaries won't do well, you always should have an artillery squadron of your own, say like 5 to 10 artillery units per 20 mercenaries at least, depends also on your income and force limits, ensure that a half of your force limits are empty and free for mercenary use.
The reason you should have artilleries of your own is because there are very few mercenary artilleries available, and you really need heavy firepower support, otherwise you will lose every battle.

So once a war starts you simply spawn all available mercenaries and attach them to your home arty squadron, put there a leader and send them to an offensive march and you can be totally careless about manpower. Just attack, be offensive (but only on flat terrain!) don't wait in the Alps or in your home territory for the enemy to come, you have to keep him away to avoid his seige! Use these mercenaries as much as you can and don't spare any of them alive, They should not come back to home territory once left! Use them for an offensive purpose until they lose like 70% of their total strength.

Suppose you have lost most of your mercenaries (around the 70%), then immediately detach your arty squadron and send them back to your capital with the leader, disband all the wrecked mercenaries you got there in the enemy territory and then again respawn new ones in your homeland just after a month (you have to wait until they become available soon with full strength). Attach your arty squadron and go attack the enemy again and again, until he loses all his manpower, gets war exhaustion and then you can siege him faster and win the war without much strategical effort and resource lose (manpower\diplo points etc...)

I just tried this intensively with small trade nations\ HRE electors and mediocre colonial nations, and this is nearly always working, unless the enemy is directly attacking and sieging your homeland, then you have to have two separate chunks of land by a strait or a huge distance, Like say Austria+Low countries, or Anatolia+Balkans (Ottomans), or even Sweden+Finland and the Danish straits.. so you can always spawn new mercs in the other lands while the enemy cannot access them from the other side.

 

[Junuxx]

Don't disband, consolidate.

I'd only use mercenary infantry. Not only the artillery, but the cavalry too should be your own guys. Mercenary cavalry and artilery are just way too expensive. In addition, they typically don't take nearly as many casualties as the infantry, so the manpower benefit is kind of wasted.

 

[WKIII]

Don't disband, consolidate.

I'd only use mercenary infantry. Not only the artillery, but the cavalry too should be your own guys. Mercenary cavalry and artilery are just way too expensive. In addition, they typically don't take nearly as many casualties as the infantry, so the manpower benefit is kind of wasted.

I disband them because until they will refill their strength the enemy may attack this weakened army and destroy totally my artilleries. In addition they suffer a great attrition.
From my experience, the cavalry are dying like a half less then the Infantry does, so yes it is a significant manpower lose.
Well yeah, the cavalry and artillery are very expensive by default. But once you have researched the Administrative ideas you get nice discounts on them, on both hiring costs and maintaining (overall like two thirds cheaper deal), so they are still affordable.

 

[Homer2101]

Consolidation is more efficient than disbanding, unless the AI ignores pure-artillery stacks. Which would be a programming oversight and probably should be patched.

There is usually no reason to use mercenary artillery. The primary advantage of mercenaries is that they do not draw on manpower. Artillery should not take any casualties in battles and your natural manpower pool should be sufficient to handle routine attrition losses.

Amount of cavalry in an ideal combat stack will depend on country and tech level. Assuming a typical Western-tech country, there should only be four to six cavalry regiments in a full-width combat stack regardless of tech level. Whether the cavalry losses are sufficient to justify the increased cost of mercenary cavalry will depend on the depth of the manpower pool, but I would usually use regular cavalry regiments and instead purchase additional mercenary infantry.