When you target a monster, one of the following terms will appear beside his level. If nothing is to the right of his level, he is soloable. Also beware of up and down arrows.
See this article for more complete information regarding the Consider system.
This monster should be a reasonable fight for a single player of the same level as the monster. Feel free to spank him with abandon.
Fighting this monster should be a reasonable risk for a group of 2-6 players.
Epic x2, Epic x3, Epic x4Edit
This monster is intended as a challenge for 2-4 full groups (12-24 players) or sometimes a single group within the level range indicated. There may be earlier steps that are soloable or can be done by a single group, but the final step will require a lot of friends.
Generally speaking, Epic Quests are some of the most difficult and challenging content in the game, and should not be attempted lightly.
NOTE the number after the x indicates the number of full groups of 6 players the encounter is designed for.