The process consists in: application through agency. Amgen hires first only from agencies.
Secondly you are asked to take a test, there are several test. Nothing difficult: language (quite easy), for instance pick the word that doesn't fit in the context.
Math, no calculation at all, just some series or easy algebra. Finally inductive/deductive test. The only difficulty is the timing, 5 minutes for each part and 20 questions each part.
HR interview, this doesn't happen always, i.e I skipped it.
Managers interviews. Deep and tough interview, about your background, your experience, personality, stress resistance, skills. It is easier if you turn all in a conversation, managers can be really kind, but they need to be sure you can resist the high level of stress that is in the job.