We used to fight skeletons by dodging their arrows and punching them.
When we grew stronger we just absorb the tiny damage they deal and give them a lot in return.
Obviously they aren't happy! So they started picking up rubbish thrown by players.

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After a long time of picking up stuff, they now have loads of them and they are now trained to:
 - run away after they hit you, then come back (purely accidental addition, may not work)
 - switch to melee if players get too close
 - switch to bow if players are within range
 - enderpearl to players when almost out of range (no running away from them!)
 - drink special potions that regenerates their health and give them some absorption.

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If you want to test this mechanic out, hit the 'download' button and play the world in 14w26c to 14w28b.

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Map Features:
 - control the amount of skeletons spawned
 - auto-equip supply chest
 - landscape for simulating battle (may change a lot)
 - kill button
 - button to change to creative and check out the redstone
 - toggle skeleton ability