The best way to kill thugs as Xian

Yes. Most achievements and challenges are cumulative across all playthroughs and modes. The achievements relating to collectibles (Gotta find 'em all and Nearly there) seem to require using a single character, but you can repeat the campaign with a character in single-player mode, which keeps your skills and inventory intact, including all collectibles you've found, so you can use that game-plus to get the rest of the items you missed the first time through.

Also, the challenge relating to collectibles (My collectibles, let me show you them!) does count total collectibles across all characters, and they do not have to be unique collectibles. (Example: If you get Banoi Herald excerpt #1 with all four characters, that will count 4 toward the total of 140; you do not have to find 140 unique collectibles across all four characters.)

Note that this gives you an easy way to level up a new character: get to 90% or more on a Level 4 challenge with a high-level character, get to a checkpoint to save your progress, then load a low-level character and complete the challenge. Because the experience reward does not scale, a reward that might not change a level-50 character much can boost a level-5 character several levels.

There are three different kinds of bonus experience in Dead Island.

The first is bonus experience from breaking limbs. This is possible via blunt weapons and fists.

The second is bonus experience from cutting limbs. This is possible via sharp weapons.

The third, and last, is bonus experience from head-stomping fallen zombies.

Additionally, this bonus experience is scaled based on the level of the zombie, and is also affected by some skills in the characters skill trees.