1) Click on him with right mouse button
2) Watch the show
3) He should be dead now

1) Get stacks. If you don't have stacks, then why the hell are you playing Nasus? 500 should be enough
2) Get double buffs (not that important)
3) Dodge his knockup and others
4) Pres R and spam Q, use E whenever it's possible, also use Ravenous Hydra
5) When he gets too much stacks on you, hit Mercurial Scimitar and you should be OK
6) He should be dead now

1) Get your Bloodthirster shield up
2) Red buff will help, but it's not necessary
3) As with the Red buff, get some Dragon stacks, but that's not that important
4) Attack the Baron withe E and Q
5) Use your abilities whenever it's possible (Q and E are most important, but you can use your R too)
6) When you get too many stacks from him, use Mercurial Scimitar
7) Don't forget to dodge his attacks
8) He should be dead now