For the most part try to get combat art bonuses from gear rather than eating runes (its half the regen penalty iirc). As for combat art level/no. of runes - its subjective. Try look at all the factors for damage (weapon lore, strength/dex, elemental damage, base weapon damage, set bonuses, etc) and all the ones for reducing regen (regen special moves, concentration and physical resistance improves it too - can get decent bonuses on gear). If you are having trouble killing monsters its usually because 1. you need better gear (vs no. of runes or combat art level) for more damage or less regen
and 2. you should make a buff suit, get some yellow potions and start using combos.
Never used the unicorn much - you could try using call of the ancestors - very strong esp if you are lucky enough to get full ettol rahc bow and set or use thorn bush - always fun when it explodes
EDIT - For a comparison, I looked up a multi shot wood elf build - Level 73 GOLD SB 87%, Explosive arrow at L9 - 1.6 sec, Penetrating arrow at L18 - 1.8 sec - damage of 23K to 30K, Combo of penetrating arrow (L32) x 4. Multi shot at 19, resistance at 4900. Weapon lore at 106. Picked trading. Araelfis bow and 3 others of the set. Could improve regen to about 1.2 - 1.3 shuffling things around and shopping, maybe a bit more damage.
At Level 55 and Silver (bonuses are much better in gold and beyond) you would want to do (as a rough estimate) 8 to 12K damage depending on your gear and set bonuses. As long as you happy with your killing speed then its not a worry.