What advice do you have for preparing for a Spanish speaking assessment?

My biggest piece of advice would be to practice speaking your presentation with your written script in front of you at first. This way, you can focus on pronunciation, intonation, and where to pause for breath. You also want to try and sound as natural as possible, so you could try listening to recordings of native Spanish speakers to help you. Once you feel confident with your pronunciation and flow, you can begin learning your presentation from memory. Break it down into small chunks so that it does not seem intimidating, and practice little and often. From each section, write down a few key words which will act as your prompts in the learning phase. As you begin to memorise it, gradually take the key words away until you can do the whole thing with no prompts.
Another top tip is to record yourself practicing – this might sound strange, but you can listen back to yourself and follow along on your script, listening out for any missing words or incorrect pronunciations. You can also listen to it when you have a spare moment, to make sure you keep it fresh in your mind in the lead up to the assessment. Also, try to pre-empt some of the questions your examiner might ask you. Try presenting in English to a friend or family member and get them to ask you questions to give you an idea of questions an examiner might have. Then you can revise vocabulary to help you with these too.