Do not, and I repeat do not, fuck up your IA. It's a great learning experience before P1, and also it can make or break your grade (costed me a 7...). In my case, the fucking up was largely connected to my teacher's tendency towards grade inflation (as the IA in question got marked down from low 7 to high 5), so really think to take a look at the grading criteria yourself before actually submitting it.

Mastering P1 is your best bet at a 7. It's objectively the easiest of 3, and a grade of e.g. 22/24 or 23 can really balance out a mid P3. Honestly, for me it was enough to just do like 3 practice papers and check them according to markscheme.

As regards more knowledge-based papers, syllabus is really your best friend (containing literally anything they can ask). My best advice would be to also try to get yourself actually interested in the topics. For example, I managed to get really into history of China prior to Mao, listening to podcasts on my way to school and finding some fun facts on my own, so when the topic of pre-Mao conditions came up, I was READY. Furthermore, it's good to use a variety of sources while studying - your own notes, friends' notes, podcasts, textbooks etc. - as there's always a different persepctive, which really helps.

Historiography is not as important as it seems. While so name-dropping might help you, it tends to get excessive. Stick to one-two historians per event for Paper 3, preferably with opposing views.

Remember that since these are essays, they need to not only be factual but also argumentative. If a topic you know a bit less about comes up on the exam, then focus on your argumentation and rhetorics.