Score an overall AAA in Heart Attack Mode.
Finally got the Cardiogram trophy taking the GOAL A path (LEFT, LEFT, LEFT, LEFT). Wasted hours trying to get it through the GOAL D path (LEFT, RIGHT, RIGHT, RIGHT) but just couldn't get AAA for the Ice Scape stage. Switched to GOAL A and got it in 2 attempts. What a relief!
Here are some tips on how to get the trophy for those who are having problems:
I find the LEFT, LEFT, LEFT, LEFT or LEFT, RIGHT, RIGHT, RIGHT paths to be easier for this achievement. Do it on VERY EASY difficulty for lots of time on the clock.
The LEFT, LEFT, LEFT, LEFT Path (GOAL A)
1st Stage: Sunny Beach
The two challenges are easy and there shouldn't be any problems with them.
2nd Stage: Bay Area
The first part requires you not to hit anything, including the walls. Hitting anything will result you in getting AA even when you filled your heart meter. You should be able to find path through the centre with some minor adjustments. Slow down if you have to, you won't be penalised for that. The second part requires you to collect as many coins as possible. Position your car straight up the gold coins and then make a minor adjustment to the right to hit the whole bunch of coins in the middle. You should be able to get enough coins for AAA. The final part should be easy.
3rd Stage: Waterfall
The first part requires you to run through blue, which should be easy. Slow down by letting off the gas button if you need to adjust your car. The second part requires you to hit all of the blue cones, which can be tricky if you are running at high speed. Instead, I shfit down/brake to below 100 km/h just before the cones and slowly steer to hit all of them. There should be more than enough time for that. The final part should be easy.
4th Stage: Lost City
This stage is one special request to beat all the cars (8 total) in a race. Just race as normal and you shouldn't have any problems getting 1st place.
Final Stage: Giant Statues
The first part requires you to hit all the blue cones, so slow down to a crawl like in Stage 3 and you should be able to slowly hit them all. The second part is requires you to go through all the gates, which is similar to the cones if not easier. Slow down and go through the gates at your leisure. Finally, the final part requires you to hit all the cones while drifting, which is the hardest part of the entire route. Fortunately, the angle of the drift is a small 'S' and not too difficult to hit. Try practicing a few times or watch some videos on Youtube for the turn angle if you are having trouble. Clear that part and the trophy is yours.
The LEFT, RIGHT, RIGHT, RIGHT path (GOAL D)
Quite a number of people have gotten their trophy with this path. This path is an alternative for people who hate to hit cones. The first two stages are the same as the previous path.
3rd Stage: Big Forest
The first part requires you to hit enough ghosts to fill your heart meter. The eaiest way to do this is to drop a gear down to get better control of your car, letting go of the gas button when necessary. Your speed should be sufficiently fast for you to hit more than enough ghosts. It may be harder to do this on AT. The second part requires you to drift through red, which should be easy.
4th Stage: Ice Scape
The entire stage is one special request to overtake as many convoy trucks as possible. This is the hardest part of the whole route, as the trucks are big and difficult to navigate around, especially in the tunnel. My suggestion is to plan your driving line around the corners and trucks and hopefully get past enough trucks for AAA. I had a lot of frustration with this one.
Final Stage: Milky Way
The entire stage is one special request to collect as many stars as possible. Fortunately, this one is much easier than the previous one. Simply reduce your speed to mid-100 km/h and slowly collect all the stars. Switch from the left to the right path midway as it seems to net more stars. You should be able to get enough stars to fill your heart meter for AAA provided that you don't crash.
Update: A detailed video guide on taking this path is available on Youtube here. It's very well done and covers all the points mentioned above.
I hope you find this guide useful in obtaining your trophy. Feel free to suggest more tips in the comments.