:::DarkPaladin here, tips on how to kill basically every class:::
Rambo Scout.

Soldier:
Soldiers are personally, my worst enemy as a scout, Rockets are a pain
in the ***. If you are playing MegaTF and are in a relatively open area.
Fire and dodge the rockets until the soldier gets too close, then jet
jump away, and nail him until he gets close once more, and if necessary,
jet jump away. Not only will it get you the kill, but it annoys the
soldier.

Demoman:
Grenades have a limited range, right? So use that limited range to your
advantage. Fire from a distance. Easy Kills.

Sniper:
These are my personal fav kills, most snipers can't defend themselves
well enough to survive an attack. Concussion them and fire away.

Scout:
Frankly, if you can aim, you can kill a fellow scout rather easily,
'nuff said.

Pyro:
Stay away.. Stay far away, and attack from a distance if you don't want
to get your ass fried.

HwGuys:
For some reason, conc's are not very effective against these beasts. I
use flash grenades and circle them until they die off, only problem is,
it takes a while.

Spy:
Spies can't defend themselves, so kill them.

Engineer:
Beware his rail gun! It will take your ass out quick if hit by it. Use
grenades here, then nail him.

Medics:
Be very careful of medics, they have a weapon 2x more powerful than
yours, and can nail you faster, use a similar technique as on soldiers
with medics if you are in the open, if not, grenade them.


:::Ultimate Defiance:::
Let's say you have a lonely sniper on their tower, and you go to visit.
Conc grenade them once or twice, then whip out the old axe/knife. Hack
away until he dies. This also works on spies and demomen (only if you
are careful).