-Reasons/Insight-

1. Crit doubles nasus Q damage, meaning that at 400 stacks (at 20 minutes roughly), he'll be dealing 800 damage ON STACKS ALONE (that means one-shots, yey);
2. He has 20% free lifesteal on his kit meaning that he doesn't really need anymore, letting him free to build 100% crit and heal lots of his crit Qs;
3. If done correctly, can one-shot squishies anytime late-game, and since nasus benefits amazingly from stormraider's surge and move. speed items such as youmuu's, he'll be unkiteable alongside with flash and tenacity from merc. treads and elixir of iron (if you choose to buy it);
4. Crit nasus has the BEST (quote me on that) 1v1 potential late-game with his one-shots and mobility leaving the enemy carries no outplay potential.
5. Feeding? Doing bad? 0/10/0 early? Just farm! You'll still carry anyway.
6. "No Sheen? You disgrace Nasus! Sheen has so much synergy!" Nope, and you know why? You'll get get about 200AD from items when full build, and that becomes 400 raw damage on Q from the crit proc, meaning that it's much better than a 200 damage trinity proc, also, you don't need trinity since you got youmuu's and stormraider's mobility.

-Tips-

6. Try to take at least 3 stacks from chikens/krugs and 3 stacks always from wolves. Taking every single stack from those multi-monster camps will slow your jungling by a lot because of the cooldowns. Also, don't forget about the scuttlecrabs which give 6 stacks each and dragons (10 stacks).

7. Preferable jungle route:
-Red buff (Smiteless/Can solo)
-Chickens (w/ Smite)
-Wolves (Smiteless)
-Blue buff (Smiteless)
-Gromp (Smite if needed)
-Scuttlecrab (If up)
-Recall/Gank.

8. Preferable bans/scary opponents:
-Lee Sin (BAN HIM ALWAYS WHEN YOU CAN - immense 1v1 potential on jungle/river, can steal your buffs super easy early, 1-shot you @6, pop your ulti when he's about to combo you)
-Shaco (can stealthly steal your first blue and will always try to fight you @6, pop your ult instantly against his burst, do not fight shaco if you have been ignited by him)
-Ivern (can cheese-steal your buffs, ward and beware, be sure to steal his buffs while he's stealing yours if it happens but beware of his Q's CC)

Hope you like this guide, it's wildly powerful

xoxo, GinWinsky, BR Servers \o