Summer Cup for HoMM 3 on the TE-lite version.

Considering that the generator and the value of monsters in SoD and ToG are the same, it seems that there is a lack of observations. At 6000, 5th-level monsters appear quite often.

If you mean the Guardian of the Grove, I remember the following encounters in SoD:

Corrupted Liches;
Thunderbirds;
Hellhounds;
Corrupted Minotaurs.

I haven't encountered 5th-level creatures from Castle, Fortress, Rampart, or Tower factions in the Guardian of the Grove encounters.

If you mean simply guarding the Zone, then I encountered:

Pit Fiends;
Dryads;
Monks;
Earth Elementals.

I haven't encountered any other 5th-level creatures.