There are 12 different combat scenarios you can take part in, each with different objectives. One you take first place in all of the first eight missions, you will unlock the 4 ‘advanced scenarios.’
There are four types of missions, Time Trial, Capture, Survival, and Hunt. Each individual scenario has a set requirement in order to place first in it. The requirements to gain first on each scenario are as follows.
Time Trial (Warehouse) - 01:24:67
Time Trial (Tropical) - 01:20:94
Time Trial (Volcanic) - 01:54:61
Capture (Volcanic) - 02:00:53
Capture (Tropical) - 01:58:67
Capture (Subterranean) - 01:29:47
Survival (Volcanic) - 01:31:83
Survival (Tropical) - 01:46:43
Survival (Subterranean) - 02:20:57
Hunt (Subterranean) - 35
Hunt (Volcanic) - 43
Hunt (Tropical) - 50
In time trial, you must kill all the enemies within a certain amount of time. Capture is similar to time trial, but instead of killing everyone within a specified time, you must stay near each point and capture them with a certain amount of time. Survival, you need only to survive for a certain amount of time, with it ending when you die. Hunt requires you to kill a certain amount of enemies within a certain amount of time.
All missions (not the individual trials) found here count towards Completionist and all of the Ally achievements.