The tips I'm about to tell you are what I used myself. The following helped me notch 15 700k+ games and consistently hit around 650k. Just for your information, according to the leaderbord I am seeing, I am currently second place in terms of score with a score of approx. 850k.
Anyway, here were go!
(1) Cloud Temple 3 - This is by far the easiest and best level to do it on. There are three circles: one small core circle, one medium inner circle, and one large outer circle that contains all the mini-boss statues (the objective statues you need to go destroy the boss statue). To perfectly execute this level, you will want to drive down into the inner circle (not core) and then begin to drift in a circle while knocking out that entire circle. This should get you to a 6-7x multiplier. After that, start destroying the mini-boss statues. Do not worry about getting more time! If you do this right then you should be able to get to the boss statue with around 720-740k. FYI, the boss statue you gives you 80k (I've gotten 100k from him but I don't know why). So the goal is to end the level with a minimum of 720k.
(2) Hit pieces with the back of your vehicle - This is the most important advice you will ever get. When you're drifting, your car is essentially going in circles on its side. Well, the key to racking up a lot of points is to hit all statues with the rear bumper of your car while drifting.
I'll give you an example. Let's say you have a 8x multiplier and drift into a mini-boss statue but you hit it with the side of your car. This will only give you around 8,000 points. Now, if you hit that same statue but hit it with the back edge of your car then you will get around 80,000 points!
This applies to all statues. Mini statues will give you around 4,000-4,500 per hit at 8x multiplier.
(3) Get 7-8x multiplier before hitting mini-boss statues - You get a multiplier increase by hitting a cumulative amount of objects in a single round. The multiplier is useless, though, if you do not hit objects with the back of your vehicle while driving. The mini-boss statues are worth the most and are the key to hitting 800k. If you get an 8x before hit any boss statues and you perfectly hit all of them with an 8 then that will automatically get you your 800k score.
(4) Do not make rapid hand movements - The key to drifting is subtle and slow hand turns. The game is extremely sensitive, so any sharp turns you attempt to make with your hands will cause your vehicle to react sharply as well. Be gentle and turn slowly. You want to maintain a drift where you are going in a near perfect circle. This can only be achieved by gently straightening out your vehicle to a wider drift.
(5) Do not lose drift - If you lose your drift then you loss your multiplier. Simple.
Now, there are some things that I figured out to save your time:
(1) You can't earn this achievement in co-op - Several people on the leaderboards have over 800k in co-op but do not have the achievement.
(2) You cannot maintain multiplier going to the boss - Once you hit that launch pad that sends you to destroy the boss statue, you will lose your multiplier.
That's it. Don't give up! This took me 2 hours straight to get. Once you grasp the concept of how to drift in a circle and how to hit objects while in a drift, then this will come with time and patience. Just keep practicing and follow my rules.