The 'Mass' game mode is a compitition of who has the most mass. This is the most popular mode to be played and its servers are usually filled.
Its important to keep an eye out for enemy slugs. All of them. When you're intent on killing one slug, you might miss another one sneaking in from the side.
When you're big and on the leaderboard, you're targeted. Behemoth slugs can expect at least 3 smaller slugs tagging along, waiting to kill them and gather some big mass.
Dieing Near BordersEdit
This applies to the Team mode as well, its usually a better idea to die near a border (or even better, a corner) so you'll have a good idea where your mass is when you respawn.