It's hard to see him beating Obama to begin with and then not screwing up enough for Clinton or Sanders to win in 2016, but here's my best scenario for that:
2012
President Barack Obama (D-IL) / Vice President Joe Biden (D-DE)
Fmr. Governor Mitt Romney (R-MA) / Senator Marco Rubio (R-FL) ✓
2016
President Mitt Romney (R-MA) / Vice President Marco Rubio (R-FL) ✓
Fmr. Secretary of State Hillary Clinton (D-NY) / Senator Tim Kaine (D-VA)
Romney wins 2012 on a combination of Tea Party energy and a worse geopolitical situation (global economy crashes again in 2011 as a contagion effect from a Greek default and exit from the Eurozone, terrorism, maybe a Crimea-style Russian or Chinese power play) but doesn't end up starting a war and losing 2016 badly like he realistically would. The Romney administration is somewhat libertarian, improving latino outreach in particular, and benefits from uninterrupted economic recovery. Clinton overcorrects for Obama's perceived dovishness and social liberalism and makes the race with Sanders much closer. It's no competition when Romney's "common sense" plan for energy independence is juxtaposed with Clinton's plan for a no-fly zone in Syria.
Romney's second term, on the other hand, absolutely would be a disaster: absent the JCPOA, he avoided war with Iran long enough to be reelected in 2016, but who knows what happens when Iran and non-state allies start attacking American forces and interests in the Arab states. Whether or not he avoids that war and the economic fallout that would come with it, there's COVID, which *hot take* was a death sentence to any Republican administration because of conservatives' conspiratorial thinking, anti-masking, and anti-vaxxing. Either Romney caters to it and takes a hit outside the base like Trump, or doesn't and takes a hit from the base. The Republican primaries have an explosive grassroots revolt that declares former Tea Party star Vice President Rubio a "RINO", while the Democrat primaries boringly mirror IRL 2020: Sanders runs again but the establishment throws its weight behind the Stop Sanders candidate, except said candidate probably wouldn't come from the failed Obama administration.
Senator Josh Hawley (R-MO) / Senator Marsha Blackburn (R-TN)
Senator Amy Klobuchar (D-MN) / Fmr. Mayor Julián Castro (D-TX) ✓
That said, the likeliest scenario is Sanders or Clinton beating Romney in 2016 IMO.