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.