First, I recommend setting out what you actually want. If it’s a higher fence and proper drainage, then talk to the neighbour and give them a chance to respond.
If they say no or even look like they are going to delay acceding to your reasonable requests, tell them you will be taking it further.
Then you’ll need to get straight on to it. I think my first call would be to council. The whole drainage issue needs to be sorted out and that would be the starting point.
Then I would be check the original strata plan to see what is actually lot property and what’s common property – and taking discussions with the committee from there.
Finally, I would be looking at strata law and your by-laws to see if there’s anything there about changing lot property so that it’s no longer in keeping with the rest of the building.
And perhaps explaining that you have an action plan will make them a bit more amenable when you restate your request.