These hidden achievement/rep rewards are the most infuriating thing Ive seen them add to events.
The things you need randomly spawn on certain maps. The goblins can spawn randomly on the 5-S1, 5-S2, 5-S3 or 5-S4 maps on the Summer Reingar map. The scorpion chick can spawn on 9-S1, 9-S2, 9-S3, 16-S1, 16-S2, or 16-S3 on the underwater map.
If they spawn, they will always be the second fight on that map, so you can enter the map, clear the first fight and look at the minimap. If you get the boss skull icon on the minimap, then the goblin or scorpion will be there. If it's the crossed swords, that's a regular fight. If the goblin or scorpion didnt spawn, yield out to save energy and go to the next map. You cycle through all the maps they could possibly spawn on checking that second fight. If you check them all and dont see a spawn, you need to clear a map (any of them, doesnt matter, do an easy one to save energy) to refresh the spawns and do the rotation again.
If you see the goblin or scorpion on easy difficulty, kill it, clear the map then move up in difficulty on the same map and there's usually a good chance they will be there too. It's tedious, annoying, and fucking infuriating to clear.