If you're the agressor (which you should be), I find the best way to approach things is to start from outside map range and charge in (w/ teleport) and as quickly as possible tele-ww stomp the necro, making short whirls all over their face until they get away or die. This minimises the number of in-range BS that might re-target you while you are WW-ing on them, which is possibly your biggest problem when attacking. Like luis said, don't try anything big headed w/ a stream of BS chasing you because they will probably catch you. Use bushes/walls (especially external corners where the two walls overlap each other and are useful as shields) to kill off bone spirits chasing you so that you can re-launch your attack w/ as little bs interference as possible.
Another important factor is proper ww length, don't do anything over 2 or 3 character widths or you'll be in WW too long to avoid an incoming BS should the necro get out of your WW quickly. Most necros play very poorly when paniced, and its easiest to achieve this by hounding their asses as much as possible. The more they're walking/teleporting to avoid you, the less they are firing off spirits/spears/teeth. Closing down the range is your only option, you can't really play defensively against something with such huge range.
A good necro will keep (or at least try to keep) a pretty good distance from you and use their range advantage, so you really have to shut that down. Don't worry about clay golems or decrep, it just makes your ww much more effective when you are on them and ww-ing. If you are slowed to this extent though, don't try to avoid BS chasing you by running as it will probably be faster than you, try leaping or teleporting instead. Leap is nice to get out of bad situations since it goes over projectiles, doesn't get slowed down, and can be executed while in hit recovery. The only really "bad" curse is iron maiden when the necro buries himself in bonewall, all I can really advise there is use zerk accurately. This is also pretty much the only way to punish a necro that corners himself and blockades himself in with bone wall/prison (HELLO STEVEN) the only bad thing about zerk is its relatively slow (compared to ww) and is a bit of a crapshoot versus a 75% block opponent. Having said that it can one or 2 hit any necro,
which is most satisfying.
