At our school we utilize the line drills at every belt level. But we have a lot of variants that we use and I will tell you a little about my favorite one. It is a version of the mass attack drill that goes like this.
You have 4-8 people (the attackers) in a circle around the 1 person in the middle (the defender). The attackers attack with any type of attack (punches, kicks, grabs, choke holds, Multiple attackers at once, or with weapons like the stick, knife or gun). The person in the middle must deal with the attacks the best they can with their known techniques, they may add endings to their technique if they want. Or if blending/borrowing is preferred that day the defender has the freedom to do as they please as long as it satisfies the #1 rule. IT Must Work.
We have to reach a level in this drill to advance to the next belt. At blue belt you need to do this drill at a 16 out of 20 level. You can make only 4 mistakes. A mistake is hesitation, technique that doesn't work, or done on the wrong side. At green you need 18 of 20 and brown and black you need to be perfect at 20/20.
I love this drill because it is an intense drill that, if done right, can bring you as close to the street as it gets. It builds reaction time, eliminates hesitation, and teaches you to attack with total conviction.