@The_imp
I also use the oil+ fire and rain+bolt combo, but they enemy is still kicking my butt - I had to turn the difficulty to normal lol.
I'm still in Cyseal with still a lot of quests left to do. Which is a good thing

What level are you?
I find the different arrow heads offer enough variety to a marksman.
Set an enemy on fire, then slam a poison arrow into him for a blast. Or shoot a fire arrow into a poison zombie and watch it go boom.
Electrify groups of enemy in the rain, shatter armour then watch the knight's giant two handed axe swing for 150 damage (no joke, Madora hits hard!.. or maybe because of the axe)
The marksmen skills seem to favour a more supportive play.