Making Halsin a companion for a level 1-4 party would completely undermine his character and archdruid position in the game world and narrative.
It's comparable to making Galadriel a party member in LotR even though Halsin isn't quite that epic. To build a believable world and have some scale of power in it some monsters and NPC's have to be above and beyond your PC's. An archdruid definitely fits this bill. He has power and knowledge far beyond you and is able to teach and guide the party, like Halsin is doing. He has responsibilities leading a circle of druids and no time to join whatever rookie adventuring party that just showed up.
Halsin isn't actually that powerful. You get to kill him as part of the evil storyline (depending on the choices made). Which is a tough battle, but doable.
