Gaining Bushido skill after 90 was very easy for me by following this method:
Buy or craft the weakest (least additional item properties) 100% poison damage repeating crossbow you can. I have one with only 25% Stamina Leech. Sail a boat to Wyvern Island and park it just off the coastline. Lure as many Wyverns as possible near you. Remove all gear to your backpack. Proceed to fire at the Wyverns using Momentum Strike. You can gain up to 120 with this method and you barely do any damage to the monsters. However, you must keep their health bars up in case the *your bow does extra damage* piles on too much bonus damage. If you notice that any Wyverns are running low on hp, just swtich targets or even stop attacking and relax for a minute. They heal very quickly and you can be back at it again in no time at all.