The hobo merchant will not sell you the goth clothing just yet, that comes later along the main quest, on the second day after the announcement of the the new Taco Bell at the South Park mall, Cartman will tell you that the elves stole the stick in the middle of the night, and that the humans will need reinforcements from the goth kids.
Deliver Cartman's message to them, and they say they'll only help you if you look and act like them;
- To look like them, you'll need the goth equipment from the hobo merchant by the U-Stor-It, which he should now sell. (Allegedly, you only need the Goth Cap, Goth Clothes, and Goth Gloves equipment. Any goth hairstyles, glasses and makeup are supposedly optional. But, you might as well go for the full look.)
- Coffee - Head to the Tweek Bros. Coffee Shop and buy some Tweek Bros. Dark Roast. It is the only coffee dark enough to be considered goth. As a bonus, this will also make you friends with Tweek's parents.
- Cigarettes - There'll be some tough looking kids in the alley beteewn Jimbo's Guns and the bar. Beat 'em up and take their smokes.
At some point during this process, the elves will kidnap you and take you to their hideout, behind Kyle's House. They'll try to convince you to join with them, not Cartman, and recruit the goth kids for the elves. They'll let you go, you can decide who to ally with later (And there's a few friendship opportunities here).
Return to the goth kids wearing your new goth equipment, and they'll send you on another quest - to get a photo of yourself with an anti-conformity sign in front of the PTA. Various bits of story occur, and Randy will teach you a new spell, and sends you to investigate the Taco Bell before he'll let you take your picture. This leg of the quest will require some combat, so switch out of your goth gear. When you return with evidence against the Taco Bell, Randy will take your picture and send it to you (Talk with the PTA milling about after for another 5 friends).
Re-equip the goth gear and show the goth kids the evidence of you conforming to their non-conformity (If they keep telling you to get that picture at this point, you do not have your goth disguise fully equipped, I got stuck for a while here because I had some other black glove equipped). This isn't quite enough to sway them however, you must now show them you can dance like a goth. You must satisfactorily complete a brief DDR sequence - NOTE: Myself and a number of other PC players got a little stuck here, the DDR sequence is not controlled with WASD, but with the arrow keys. No amount of trying to use WASD or trying to out-think the game by not conforming (not pushing any buttons) will get you past this.
Now sufficiently convinced of your non-conformist ways, all four goth kids will friend you. Now you must decide who will you report their allegiance to.
With respect to targeting this achievement, there isn't a huge amount of difference between the classes.
The one thing that may significantly impact your quest for this achievement is the fact that if your buddy dies, if you can revive them before the end of the fight, you're still on track for the achievement. What that means is that you may want to build your character and select skills in such a way as to minimize your use of debuffs or Damage Over Time effects. (This has the side effect, incidentally, of making much of the game much, much, harder, for what it's worth, so I'm not sure I recommend going all in on the strategy - easier to reload, IMO.) This will allow you to make combat a bit more predictable, at the expense of it lasting longer. Incidentally, the class best suited to avoiding the application of DoT's - the Fighter - is also the class with access to the widest variety of abilities that can lower your opponents Attack, which is also useful for pursuing this achievement.
Another option is to focus more on preventing foes from acting. In this case, the Mage has access to a Stun from right near the beginning of the game, and later, gains access to the only innate Slow effect - which can be very useful.
Finally, the Jew and Thief both have access to abilities that can apply 'Pissed Off' to targets; this functions as a taunt of sorts, forcing enemies to attack your character instead of your Buddy. (Do note, however, that the Jew gains access to this at a much lower level than the thief).
Really though, all of these differences are pretty minor. What matters more is your consumable use, and willingness to reload if things do go a little pear shaped. Every class has the ability to do lots of damage, and once you get to a high enough level to kill a mob in one attack, (the Sucker Punch perk helps) the Swat Helmet and some smart use of your 'Shoot' skill on the map will mean that you can end many fights before an enemy even has a chance to try to hit your buddy.
Best Answer
After some testing I have confirmed that this is elemental damage, however it is not extra elemental damage. This stat seems to reflect the weapons natural damage element.
I took my Staff of Winter (+50 frost) and attacked an elf that was weak to frost damage, and he took additional damage as if from a frost attack. During this combat sequence I had no strap-ons equipped to my weapon, nor did I have any active buffs or anything else that would have added elemental damage to my attack.
Note: My weapon did not do X normal damage and then X frost damage, it just did X damage, and a WEAK icon appeared near the floating dmg numbers.
I can only assume then that the +50 Frost means that 50% of my damage dealt with the weapon is Frost Damage. If anyone has any ideas on how to test this more extensively they would be very welcome.