When you target a monster, one of the following terms will appear beside his level. If nothing is to the right of his leel, he is soloable.
See this article for more complete information regarding the Consider system.
Solo
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.
Heroic
Fighting this monster should be a resonable risk for a full group of 6 players of the monster's level.
Epic
This monster is intended as a challenge for 2-4 full groups (12-24 players) 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.