Any melee character is going to be at an increased level of danger compared to a spell caster or a ranged attacker. So I suggest picking a ranged build of some kind instead of using daggers or any melee weapon. Maybe use spears, bows, or even Elementalist or Ritualist skills when you're going though the storyline. On my Assassin, I usually spam the elite skill Assassin's Promise and 3 PvE skills: Ebon Vanguard Assassin Support, "Finish Him!" and then either "You Move Like a Dwarf!" or Cry of Pain (from Mesmer secondary).
As for the brawling minigame, you're talking about the dungeon called Fronis Irontoe's Lair. In order to get to it, you'll have to finish the primary quests in Shing Jea Island and get to Kaineng Center when you're level 10 or above. Once there, Minister of Maintenance Raiugyon will give you a quest called "I Feel the Earth Move Under Cantha's Feet". And, after you complete that, you will have to keep doing the primary quests from EoTN until you get to Gunnar's Hold. And, in that outpost, you should complete the quests given by Kilroy Stoneskin: "Punch the Clown", Kilroy Stonekin's Punch-Out Extravaganza! (which is the quest for the dungeon that I mentioned at the beginning), and The Throwdown in a Norn Town. That last quest is important because you'll need Thunderfist's Brass Knuckles in order to do more damage and so you can upgrade your brawling daggers with a Furious mod (10% chance of double adrenaline), "Brawn over Brains" inscription (+15% damage and -5 energy), or Shelter mod (+7 armor vs. physical damage). Also, when you are brawling, make sure you have the maximum amount of dagger mastery and critical strikes that you can get, and maybe get an armor insignia that boosts your armor rating like Nightstalker or Saboteur. Lastly, go to Menu > Options > Control, click on "Action: Use Skill 8" and add a bunch of keys so that, if you die while brawling, you'll have an easier time getting back up.
Let us know if you have any specific questions
