This achievement can be unlocked by completing the "campaign"with 6 characters
After every level you can see the objectives the roast has, in order to get them you'll have to get a little lucky that A: you get the right sentences for your roast and B: your AI doesn't steal half of them away in his turn.
Overall, all you can do is just try a level over and over until you get it, which is probably not the answer you where hoping for.
On a positive note, you don't have to get all challenges in one go. 2 tricky challenges and you can win the game completing one of them? Great, now on your next attempt that won't be an issue and there is one less challenge to plague your mind
Good Luck and happy hunting
Edit: EpicMount mentioned the great way to get the 2x combo challenge. When making a sentence use the "and" note to make big sentences to get a better chance to get a combo. use the "..." to lengthen your sentence and make it more likely you'll get a combo. after all if your sentence starts with: "You and your mother and your uncle and your sense of style" then there are 4 possibility's for your combo in the next sentence.