In answer to your question: You should really only focus on one element at a time, and switching is easy since you can re-arrange your points in town. If that doesn't help then I probably misunderstood what you're asking.
And in relation to water magic: Water is a really good support element, I love playing water in PvE.
Air is the useless one in PvE, it has a few support elements but in general Earth or Water are better support and spikes are pretty pointless on monsters. --- Signature: Hello.
There is no element that is useless in PVE.....though fire and earth are more useful in a wider variety of areas.
The nice thing is that Sup runes are cheap now....so, do what I did and build your ele to run all 4. I literally carry 4 sets of headgear and a weapon set for each element. That flexibility keeps my ele fresh - rather than just clicking nuke chains all day.
If you have to prioritize - in order of PVE usefulness - I would say Fire-Earth-Air-Water.
Best of luck! --- XBL GT: Loki ISP....GW IGN: Nentslye Woodmaster, Pope Woodmaster I, Silussa Mindraper Clan Leader, XBL Clan Wolfpacklan...www.wolfpacklan.org