Medal Farming Notes:

An NPC takes 8 min to regenerate full defences. Leave an interval of this time before attacking it again for the best chances of getting another medal.

Valleys do not need time to replenish after they have been abandoned, and can be attacked repeatedly (they have no regeneration time). I generally find leaving 10 secs between waves gives me plenty of time to enter and abandon before the next wave hits. If the server has severe lag, you should extend the intervals.

Attacking with an Intel Hero increases the medal drop rate. If you are not taking losses with your current attacks, changing the hero armor from attack to wisdom may improve your drop rate.

Ballista counts needed change with the hero level and HBR/Archery research.
Read http://www.theevonywiki.com/index.php/NPC#Farming_NPC.27s_for_Resources for definitive information.

Drop rates: After recording more than a thousand hits, the drop rate appears to be precisely what your achievements buff says it is. This rate includes not only medals but scripts (mich/LAS), Hero Hunting, Speakers, Materials, etc. Drop rate is a total number ie while my total drop rate on npc5s was 4%, the drop rate for Honor Medals was ~1%.

Persistence counts. It may take you 100 hits to get one of the medals you need but the other medals/scripts/gems add up as well. Then you may hit two in 25 tries, but no promises. If you don't have the patience for this, evony will gladly take your money.

All medals may be found at higher lvl npcs than listed if the npc loyalty is low.

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Cross Medals
Level 1 valley.
Use hero min lvl 25 and >100 attack.
Use 50 archers or more to get minimum losses.

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Rose Medals
Level 2 valley.
Use hero min level 25 and >100 attack.
Use 100 archers or more to get minimum losses.
Level 1 NPC
Use 50 bals or more for zero losses.

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Lion Medals

Level 4 valley.
Use a hero min level 25-35 and >100 attack.
Use 400 archers or more to get minimum losses.

Level 2 NPC
Use 100 bals or more for zero losses.

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Honor Medals

Level 5 valley.
Use a hero min level 25-35 and >100 attack.
Use 800 archers or more to get minimum losses.

Level 3/4 NPC.
Use 250/350 bals or more for zero losses.

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Courage, Wisdom and Freedom Medals

Level 5 NPC.
Use a hero min level 35 and >100 attack.
Use 450 bals or more for zero losses.

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Justice and Nation Medals

Level 10 NPC
These medals cannot be easily attacked. Please see my post about medal farming a level 10 NPC for further information.