Low levels, often you can just gear for damage and just lag them to death.

If the form seems to be dodging a lot, you can lower their dex but remember that shapeshifters get haste (which increases dex). As well as a smart shifter gearing for their prime stats (which happens at higher levels). Just lowering their stats wont always do it. I found that the best specs against shapeshifters have been mace and dagger for me. Things get harder at higher levels (with my axe, mace spec, I only had one shifter that didn't seem to be affected at all by my maldicts).

This also depends on what class or spec you are playing too.