After submitting a cover letter and CV, there was maths and personality test online. This is similar to other consultancies but not as difficult.
After this, there was a written paper on a communications campaign / client / challenge.
Next, an assessment day, with 2 written tests, a repeat of the maths test from earlier, a face-to-face interview, and a group presentation in which we had 1hr to prepare and 10 or so minutes to present.
The final round was a presentation on the topic of the written paper from earlier in the process to some senior people.
It was quite spread out so it never felt too much in my opinion. Around the assessment days the way they communicated was a bit odd, ringing you to invite to the days a few days before but always at the end of the day so if you missed the call you couldn't call back to ask questions.