Make it a point of adventuring with people who aren't just the same guild. You will need someone with a lot of hit points, commonly known as a "tank" (a Fighter, Ranger, Paladin, Jomsviking or Fallen), to take the damage, a spell caster to do damage of some type (a Mage, Bard, Psionicist), and someone to heal (a Biomancer, Druid or Templar). If you want to be especially careful, you'll want an Abjurer to cast protection spells and ensure your tanks survival. Don't know a tank powerful enough to withstand the forces you wish to battle? Consider dual-tanking, wherein two tanks stand in the front row.
A three party team is the MINIMUM you should take against anything that's over your level.