Yeah Hydro is great as well but you'll want a point or two in most magic trees for the buffs anyway.
Lvl 1 Fire for Wildfire (lvl 2 for the +2 perception boost buff: Burn my Eyes)
For max range sniping you might want Farseer (+5m range buff in Aerotheurge). Lvl 2 for the Become Air buff
Rain with an electric bow is great. As well as midnight Oil with a fire bow. Personally, I wouldn't want to go without Bless anyway (Geo +30% hit buff). So 1 point in Geo is never a waste.
For lategame, once you've got the Elemental Ranger talent you can switch over to rain (To do water damage when they stand in water) but that also means that if you want to mass shock; you probably have to get rank 5 Air and pick the lightning immune talent (or use shields) to protect yourself from shocks.
It's a very strong combination, probably strongest in terms of damage for a ranger lategame, but it's pretty skill point intense. Plus you get Elemental Ranger pretty late.
Early game I think Oil is easier to use, cheap to cast and doesn't depend on a bow much. Hell you can throw candles into the oil if you want too :p
It's mostly the +50% damage boost from Bully that you want. The debuff is just extra on top. And compared with Slow Current the Oil is 1 Ap cheaper and has AoE, plus with it you can create bottlenecks that the enemy warriors try to avoid (or don't move at all).
