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.
Making him low enough level for an evil party to kill in combat isn't good for the credibility of the game world. It's as lazy as level scaling wolves up to 10 so that they still offer a challenge for a 12th level party.
If I was DMing Halsin would be much higher in level like an archdruid of a circle would be, at least 11+. The evil party would have to play dirty, cheat and poison him or something before having a chance in a fair fight. Or they would have to receive help from an evil faction who also wants the Circle gone.
Point is, there are narrative ways to keep things consistent and balance the level differences. "Halsin's revenge" isn't great writing as it is.
