Yaris

ALL credit goes to Elmore1, who posted this information recently (01/11). Much thanks to him and friend Dead Rez for discovering and sharing this huge sanity-saving exploit. Using this method the achievement can be obtained in as little as 5-10 minutes, depending on skill/luck.

NOTE: This method requires two people.

NOTE: It has been reported this does not work using the hatchback (starter 3-door car). I have not tested to confirm but the recommendation is to use the 4-door S sedan model with 14-inch steel wheels. The idea is to use a slower car to assist in getting "stuck" on the walls of the spiral in level 8, which is the key to using this method, so upgrades work against the goal here.

Step 1: Start a multiplayer game on track 8, have boosting partner join/quick-search.

Step 2: Go through the track until the City Transformer boss appears after the last lap. Do not kill him, thus giving you all the time needed for next step. One player can begin "getting stuck" right away optionally.

NOTE: MAKE SURE you have at least 20-ish coins BEFORE NEXT STEP (coin inventory is displayed in lower-right corner of screen underneath Toyota logo) because coins are needed to emit a yellow "aura" (see Step 4).

Step 3: Have one player get "stuck" on a wall. It doesn't matter who but should probably be the least skilled/familiar with getting stuck for efficiency.

Jerkkhammer's advice on getting stuck:
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"Getting Stuck - There are at least three parts of the spiral that you can get stuck in. First of all, stay on the right side only. Go as slow as possible and when the spiral is coming up move close to the right side. When it works you should get "sucked into" the spot just at the very beginning of the spiral, almost like a magnet. Immediately let go of the analog stick.

If you miss the beginning of the spiral there are 2 more spots in each spiral where you can get stuck. You'll be able to tell where they are because there will be a very small gap in the white wall. Use the same technique. Go as slow as possible then move close to the wall. When it works it should suck you right in, then immediately let go of the analog stick."
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You will have better success getting stuck by moving along the wall smoothly until close to your target versus holding the stick right down and repeatedly bouncing into the walls. Just stay close enough to avoid rubbing until your spot comes up. See Youtube video links for visual aid. Practicing getting stuck in single-player is also MUCH quicker.

Step 4: Have the other player race around the track, leaving the boss alive, until he can get "stuck" in the EXACT same spot as the first player.

NOTE: Where a car "is" will be deceiving as the cars appear to warp (as seen in Youtube video) but rely on communication to ensure you are both in the exact same notch on the spiral wall. This may take multiple tries and up to an hour. You can take turns and use both stretches of spiral on the track to reduce time getting both people in the same spot. Once you are in the same spot both players hit "Y" button to emit your yellow aura and watch the score FLY.

Step 5: Once you are both stuck in the exact same spot on the spiral and have the yellow aura activated (Y), then both players money/points will sky-rocket to 999,999.

NOTE: As soon as it is bouncing around 990K-ish, I recommend having one person leave the spot and finish off the boss ASAP. Do not remain in the spot too long or coins can start draining, but try not to leave too quick either (I watched my coins drain from 900K to 600K the first time I tried and left early in panic/excitement). Press "A" before leaving to deactivate the yellow aura, which may or may not prevent losing coins.

Step 6: Destroy the boss. Achievement pops at menu screen with very little delay and "Leader of the Pack" is displayed on race results.

Enjoy and once again, ALL credit to Elmore1 for sharing. Credit to Jerkkhammer for an excellent description of Getting Stuck, much better than I could take three pages describing it. Credit to XP1 for providing video links (never watched them, just confirmed). Credit to dawpa2000 for making videos! Credit to CsK LuCiFeR for pointing out the existence of Elmore1's thread. Credit to the "other" site (x360a) for existing.

If you find anything in this solution lacking, please take a moment out of your busy schedule to comment and allow me an opportunity to improve BEFORE voting negatively. Happy boosting!

Update 03/07/11: Added credit to dawpa2000 for videos. Yet another negative vote with no explanation, sigh...

To add to JLogan27, make sure the 'piggy banks' in the level and the 'fire hydrants' are SHOT. The piggy banks give you MUCH more when collecting the coins that come out of them rather than collecting the pig itself. Shields in fire hydrants are also valuable.

I was getting 20-23k a go (5-6 minutes) so you can imagine how much easier it is to get 1million that way.

UPDATE: The Level you want to be repeating is Level 5.