The bonus mission 'Obtain 5 Rare Ergs' can be tricky. To maximize your chances of obtaining erg crystals you can play on hard with a full party, destroying any items you come across (pots, barrels, boxes, pillars, fragments of items, etc.).
Erg Pots: two to four dependent on the map variation.
Area 1 Path 3 has two pots.
One is in the northeast corner of the first large room, behind a pillar.
One is behind a breakable wall on the east side of the second large room.
Area 1 Path 4 has one pot on a high ledge in the middle of the rolling pillars trap. A secondary weapon or ranged attack is needed.
Area 2 has two pots.
One is in the "spider pit" accessible by jumping off the ledge at the Southern extreme of the map.
One is at the top of a flight of stairs on the South side of the bosses room.
There are one or two Wisps hidden in the Battle.
Area 1 Path 1 has two at the second turn from the entrance.
Area 1 Path 2 has one in the first room after the entrance.
Area 1 Path 3 has two, one guarding each Erg Pot.
Area 1 Path 4 has one below the archer tower in the first large room.