I would try and get some time in a sim in the type that will be used for the assessment. Failing that, some time on a boeing/airbus of some type, delete as appropriate.
The main thing to remember though is that your assessor will not be expecting perfection. The ability to learn is important, and so if you can show improvement throughout the session that will go some way towards a successful outcome.
However, a sim assessment is about looking at more than just your manual flying skills. They will be looking at you as a person, assessing your personality and how well you work with the person in the other seat. Are you argumentative/dismissive or are you collaborative and open to suggestions? Do you prioritise ANC, make time for decision making, consider options/alternatives, are you calm and methodical under pressure? Do you use all the resources available to you to ensure a successful outcome? In other words how good are you at practicing modern CRM. This is more important, in my view, than whether or not you fly a perfect holding pattern or ILS.
Good luck!