Best "Very Rare" gish item?

[Osuniev]

At the level you are getting the horn, 14 CR 2 creatures are more a nuisance than anything else. AoE will make short work of them (two fireballs are usually enough, or just one hypnotic pattern)

I mean if the caster of hypnotic pattern has a DC of less tan 21, SOME of them might make the save and then start waking up the others.

On average, it takes around 3 fireballs to kill a Berserker if they fail all their saves. In the meantime, 14 attacks a turn with advantage for 1d12+3 ADD UP.

Of course, if they can, the opponent should use Wall of Force, or fly up.