From my own experience, if you are not doing pvp, water/electric works a lot better on a light witch than fire does. Sure, fire has the upper hand in Closed Lot grinding areas, but leveling up is so easy with Fiesta and CT, that I honestly don't see much of a point in it anymore, unless you are trying to level very quickly.
If I were you, I would instead pick water/electric. You will kill bosses a lot faster using Shard + Tesla + Electro + Thunderbolt. Shard, at mastery, will not fail often at all, unless your LK sucks compared to the boss' (like using 200 LK against the dragon). You should ideally be using ~320 LK for the 60th floor boss and 400+ LK for the 72nd boss. While a 3 second window doesn't sound like too much, Shard also has a very quick cool down.
Shard is most definitely worth going water for, as long as you are also electric.