How to win a level 20 cap garrison
Ok. Here is how to win a lvl 20 cap garrison, with virtually no deaths either in the alliance or with the NPCs. Here's what you need.
An alliance - full. It can be done with probably as few as 12, but if you are new, take a full alliance until you get the kinks worked out. This alliance should contain:
1 WHM in each party and 1 extra WHM in one of the parties - 4 total. The extra WHM should focus solely on curing the NPC fighters.
1 Nin/war - his job is to, at the beginning of each wave, grab the spellcasting type mob with voke, and run away. You need to grab a spellcaster (like an orcish cursemaker) that is not engaged, right from the beginning, and run out of sight of the rest of the mobs. That ninja will blinktank the mob, and not fight it. Keep it alive while the rest of the alliance kills the rest of the mobs in the wave. Continue to blinktank it while the mages cure the alliance and all the NPCs to full, and then rest MP to full. -Then- he can bring the mob back to camp, finish it off and start the next wave. This way you are fighting with full HP/MP for each wave. Make sure your meleers all have provoke, to keep the mobs off the NPCs, but otherwise you should win easily. Using this strategy in West Ronfaure allowed us to be victorious all 3 times, with at least 15 NPCs remaining each time. This strategy was discovered by Joemidnight, and he is excellent at being the blinktank.