Forza Horizon 4

Hi guys, maybe this will not be advice, but rather a recommendation. And I apologize in advance for my bad English.

OK..., Before you join to the Eliminator session, first set 4 phrases for a quick chat: Too fast for me / Hold up, mate! / Gimme a chance / I need some influence. After that, join the session and use MINI. Find the opponent on 5 levels higher than you and start the battle, and then start using these phrases throughout the battle. There is a small chance that your opponent will guess what you need the 'Underdog' achievement and allow you to win.

It helped me when I used MINI and my opponent used the URUS. I used a quick chat and eventually my opponent guessed that I needed this achievement and stopped chasing me and let me win.
Today I helped my opponent win, I used BENTLEY CONTINENTAL and my opponent used MINI. I quickly reached the final point, but stopped near it and waited for my opponent to beat me and get his achievement. After that he thanked me for my help via xbox live chat.

I hope that this recommendation will draw attention not only to those who are just going to get this achievement, but also to those who already got it. In this case, there will be more mutual understanding to obtain this achievement.

Use these 4 phrases at the same time, if you use only 1-2 phrases, then in this case, do not wait for help.

And I do not ask players who use fast cars to let your opponents who use MINI win every time, but nevertheless, let them win sometimes, especially if they ask you about it through the quick chat.

BUT PLEASE DO NOT USE THESE QUICK CHAT PHRASES IF YOU ARE ALREADY GOT THE 'UNDERDOG' ACHIEVEMENT. DO NOT USING THIS PHRASES JUST ONLY TO GET A BETTER CAR. IT WOULD BE A BAD AND DISGUSTING ACT ON YOUR PART.

Double Boxing strategy
(This is straight from my "Heads Up" Solution. You should get this achievement when you go for the other one.)

This is how it works:
1. Join a convoy with your dummy.
2. Search for Eliminator at the same time. There are never any full lobbies, so you will definitely end up in the same game.
3. Spawn in close to each other. (Sometimes you can't see each other from the beginning of the round. However, trust me, you are in the same Eliminator game. I think it has to do with the game not showing you every single opponent from the start)
4. Hide your main account. And hope that it will stay safe for a while. Usually, opponents are so scarce early on that no one will find you.
5. Go car drop hunting with your secondary account. Get something nice, like a level 6-7. Car drops become more frequent as the round progresses. If you can't find a drop immediately, keep searching. They also contain better cars later on in the round.
6. When you've acquired a nice car with your dummy, challenge your main account and purposefully lose. Voila, you have a level 6 and above car without much hassle.

Here's a solution that uses straight-up racing, though it's a little cheap - I like to call it strategic. I had spent many sessions wasting my time getting challenged by people lower than level 6 until I thought of this idea:

1) Keep the Mini, agility is required here, rather than going for a fast level 2 car like the GT-R, that is another strategy with less potential challengers (7+).

2) Spawn in the quarry / middle of the forest. It tends to be a little less busy from what I've seen. Check the quarry first!

3) Invest your lonely time at the start in finding a 6+ drop. Camp out (*see disclaimers below) within striking distance, but not too close. Being a 6+ car, you hope someone is going to go for that rather than bother with your fine self in the Mini.

4) When you see someone going for the drop, get rolling so that you're moving at the right moment when they re-spawn in the new car, mind the delay of course. Trigger the challenge on the way by. A head start is important, not necessarily everything in this strategy...

5) The rest is up to you managing the Mini to get out of the forest in the most direct, efficient way possible. Avoiding trees is definitely a skill, and the Mini is the best. There is a good chance your opponent will bonk into a tree. A strategic wager of a race.

*I was very lucky to find the quarry drop where I could camp at the South end of the top ring, overlooking the water (and equally lucky someone happened by and took the bait). This happened after 4 deliberate attempts. Very easy to accelerate downhill and it is the only way out (opponent will be facing the wrong direction). That's why I say check the quarry first.

*Camping: idling is maybe more like it. Maybe a little cheap, but is this achievement even reasonable? This solution is still more about your skill in the forest than the head start, but you'll definitely want the head start. The more time you spend in the forest getting good at navigating smart turns, and elevation 'hops'... you will find this skill can win it for you if you keep the Mini up to speed.