There’s no need to panic mate! I was in the same position as yourself about half a year ago. Applied to PWC Assurance/Consulting shared scheme where I got an offer and I’m a civil engineer.
Though I didn’t apply for Tech Consulting, I presume the process should be fairly similar. The nature of the questions during the AC are all based on the material provided on the day itself. Group exercise involves discussing about some options around an energy source proposal (could be different for others). Report writing also involves being provided a variety of options to a problem and having to choose one and summarise it in a report. So as an engineer, you should be fairly familiar with writing reports! Just make sure you have a basic format such as introduction, main body and conclusion. Some say an executive summary is required though I don’t think I had one and still passed! Your report should be clear and concise so don’t beat around the bush!
I’d say you should have some familiarity with business terms and also familiarise yourself with the latest business issues/news.
The final interview tends to be a friendly chat. My interviewer had a list of usual competency questions she had to run through with me. Just be yourself and show that you are someone friendly and great to work with! I prepared a lot for the final interview and read up a lot on business, economic issues and even issues affecting the auditing world but none of it was asked. Best to still prepare for that eventuality.
Good luck fellow engineer!