It's solo. It used to be the case that you could get help by having a two person mount and entering the cave with a friend on your mount but I believe they fixed that. Quest battles are hard. I would recommended bringing heals which won't be a problem as a life wizard and try to stack a few blades and hit then repeat until boss is dead.
Trying to get Forest Lord by defeating Grisletusk is kinda hard to solo . Can you get help on this quest or is it specifically solo?
Sorry, that quest is Solo. I have 2 life wizards that have had to do that quest. Your best bet is to do some research on this Boss. He CHEATS by casting a +35 Hex on you if you cast a Charm or Ward.
I don't remember if you can cast a Life Prism and not trigger the Cheat, but, it would be worth it to get one on the Boss. You should also have a couple of Stun Block to prevent being stunned by his minion in the event the minion casts Storm Lord. That minion is fairly low health so get rid of him as fast as possible.
Even though any charm or ward you cast will trigger the cheats, Always remember that he can only activate ONE of those Hexes when he hits. You might want to try using a couple of "Entangles" on him but that is up to you.
Here is a link to that boss and the array of spells he has.
Sometimes you just have to grit your teeth and use the charms and wards anyway. His Minion is probably going to use up most of the Hexes Gristletusk puts on you. If you decide to use a Feint, be sure to watch his pips and do so when he is low. Good luck