(Please diamond so others can see this too if you found this helpful)

Mob Arena:

Mob Arena is probably one of the most well known plugins, it had been around for years and is a favorite of he bukkit community. Many servers host Mob Arena on there servers for mini games, but also some host it for an all time gamemode on their server. Anyway to get to the point Mob Arena is a tricky one, but there are tactics to it, and I'll tell you how. But before I do remember, not all servers have the exact same configuration for Mob Arena, and that sometimes a server might set each round to 1 mob while the other server sets the rounds to 100 mobs a round, it varies and I'm hoping to chunk up (no pun intended) most of the sain Mob Arena servers.

Maps:

The environment of your mob genocide can decide whether you make it to round 50, or make it to round 3. Easy maps will consist of large open spaces with less mob spawns, whilst on the other hand hard maps will be narrow maps with a lot of mob spawns. When you start you may want to kind of figure out where the mob spawns are, some servers label each mob spawn with a certain block or a pressure plate. After that find a spot with a somewhat narrow lane with the closest mob spawn being no closer than 15 blocks. Now make sure all your teammates can fit in the lane, and make sure if you were to be shot by an arrow you would not come into contact with anything fatal.

Teammates:

Any teammate is a good teammate, unless they can harm you... What my point is even the most unskilled noob teammate can be useful to absorb arrows using his or her own face, but sometimes you need to step up and command them. In Mob Arena a perfect team would be:
(1) Healter
(3) Support
(2) Tanks

Classes:

All MobArena servers have custom classes, some may have plugins to help that concept, such as spells or guns. Whilst others may have just vanilla classes. Make sure you know every class before you tell people to use them, or if you use them in a giant team.

A good server that is dedicated to Mob Arena is:

Sasha.sector-alpha.net:25567

(this though has custom mechanics for classes, such as spells and stuff. Check it out and try these tactics)