A single slug impacts only one elbow at any given time (assuming the slug length is less than the straight length of pipe between two elbows). Each elbow should be its own load static load case.

In a dynamic response spectrum analysis, again each elbow should be its own dynamic load case. This is because a response spectrum analysis is performed in the "frequency domain", and "time" isn't a factor in the loading.

In a dynamic time history analysis, the load on all the elbows can be addressed in a single load case because the timing (when the slug impacts each elbow) is considered.