First of all her stats:

Hunger: Bloated Happiness: Very Happy Level: 158 Health: 398/398 Strength: 396 Defense: 396 Speed: 394 Intelligence: 414 Pet Rank: 1,756

Next her weapon set:

Blackmoons Hand Blaster DDS Red Rreign Cuirass Red Rreign Polearm Helm of Six Fangs Phoenix Quill Recycle Blaster Shinwas Temple Shard

Where should I go from here? If there an immediate upgrade that needs to be made?

I still can't beat any hard opponents except Daemon and Mechalag, if that matters at all.

I'd say that no immediate upgrade is absolutely necessary. However, to future proof your set, Truth Serum would be a staple addition as well as The One Ring (which would last you until 1000HP).

Your set is already quite good for your pet stats.

As for harder opponents, sometimes you need specific weapons or mods to make the fight easier