Id start with going to the CAB for advice but then I would take their answers and elaborate on them.
I have filled out numerous PIP forms and the ones with full information have always come back with the amount that "should" be awarded.
If you can get your medical records from your GP. You might be able to get some more medical evidence from what they are holding.
Google the PIP descriptors and points awarded and see which you score on. Answer honestly for the majority of your days. If you try and say you cant do x ie use the toilet without help, and it isn't a barrier of your condition then the rest of your answers will be looked on with suspicion (in my experience only)
Are you on medication that only a GP can prescribe? That also helps.
Send in all the medical info you have. There is room on the form regarding medical appointments and future appointments.
You can also Google the government guidelines to what they have to accept as an answer. I think its something like Government guidance for PIP. This gives the framework for filling in the form.
The standard that compares you to a "healthy" person is to an acceptable manner, repeatedly, in a reasonable time and safely.
They will also lie on reports if you aren't careful so make sure you state clearly what help you get to carry out any task.
Eg my mum has severe arthritis and cant cook any more. Her assessor stated as she was a good weight (about 12 stone) she could obviously cook - my retired Dad does all the cooking and snacks.
I have a similar joint disease and cant cook - I am slightly underweight, no remark was passed about my ability to cook and I scored the points.
Repeat yourself constantly. Take each question as if its new information you are giving them. I usually type my answers out and staple to the page with my name and NI number on each page.
Give much more information than the box allows.
Keep repeating the mantra of reliability, safely, acceptable standard, reasonable time.
Know your distances. If you can walk then know exactly how far you can. Again on my mums assessment they said she could walk 25m from her car to her front door into the living room. The actual measured distance is 8m.
If you use any mobility aids let them know, including how and why you use them. Tell them about all and any help you get.
Be a through as you can and you will be ok
Its a hard and daunting process but broken down it's not too bad. Be kind to yourself as you do it as it is mentally difficult.
Sorry this is a bit muddled but was literally a brain dump of PIP info