Archers

How effective was a group of archers against knights/foot soldiers? Was there any tactics that were developed to counter archers? Were archers held to the same training standards as other soldiers?

>training standards
Unheard of in post-Roman Europe until the 16th century. Armies would briefly practice moving in formation while already on campaign, at best. This is not to say that individual skill and fitness were not valued but there were no formal standards.

Archers were almost completely ineffective against an actual army in combat.

The distances a group of archers would need to stand back would make their arrows unable to pierce most types of protection.

Their true charms come out in the days and weeks after an engagement. They would rarely kill a man outright but the wounds they caused would.

For a large army having lots of wounded is worse than having lots of dead.

If they were useless then armies wouldn't bring them.

The lethality of arrows is highly exaggerated by media and fanboys but let's not go overboard. There were plenty of medieval battles won thanks to the archers disordering the enemy.

The thing that is often underrated is their melee fighting capability (probably because of popular media). In Agincourt for example they were engaging in melee too once enemy troops got close, finishing them off. They had another role as a light to medium infantry. Their gears were also pretty decent, not just shitty tunics and daggers (or something like that) besides their bows. I don't know if it's unusual or not, but they didn't step back (aka skirmish) like when you play Total War or (modded) Mount and Blade.

There were training standards implemented by Henry V for longbowmen when he instructed every able-bodied men to practice archery, IIRC.

I'm going to say that's complete shenanigans despite not having a source myself.
While armies as a whole might not have trained together, companies or whatever form of unit comprised of several men would have. This is the whole point of formations that where used, like the schiltron in Scotland or Swiss/German pikes.
I do recall there being basic tests to join mercenary companies, although none of which where in any shape or form difficult, like the landsknecht had you hop over some obstacles.

Arrow wounds rarely killed but they were very very common, very few troops were completely armored, and even if they were things like horses usually were not.

Archeres allowed an army to attack from a distance and were best used when protected by infantry

Their combat effectiveness (ability to kill in combat) wasn't as great as their tactical effectiveness. The fear of getting hit by an arrow was probably worse than getting hit by one, assuming you were wearing armour and had a shield.

Are you retarded? Mercenaries drilled all the time.

Period writers state their effectiveness was greatest when used in large numbers. I assume this means their effectiveness came down to sheer volume of fire to hit weak spots in contemporary armor.

Their effectiveness was also related to the level of armor of their opponents. Sources mention the largely unarmored Irish mercenaries at the Battle of Stoke Field got slaughtered by English archers while the German mercenary pikemen who were armored got off okay. At the Battle of Flodden where fully armored men-at-arms formed the front rank archery was said to be largely ineffective.

When a group of archers was caught without defense such as other infantry or stakes they were almost invariably slaughtered by cavalry. Which is why most armies sought to use stakes or infantry to defend them. Burgundian ordonnances go into detail as to how this was accomplished.

Prior to roughly the 14th century archers were typically the less well paid individuals. Crossbowmen on the other hand were paid more than say foot Sargents. By the 14th century this changed due to the English fielding archers en-masse, they started being paid more accordingly and came to form the best paid soldiers outside of men-at-arms.

Civic militias in a number of places refer to individual and group training as being done occasionally. The Swiss were known to have communal training on holidays.