You do know mero mero beam is just a single ability of Boa right? And I would even say it's a relatively low crowd control one at that. Like, she can petrify with her physical attacks. And she almost definitely has adv Coa and adv Coo. Adv coc is also a possibility, but a minor one.
And the Marines know this damn well. Sengoku even said "That women is strong".
So, the Marines, knowing all this, and knowing she has a whole country of haki proficient warriors backing her up, sends coby as the main force against her? What's the logic in that?
I think they could have send Gion (momousagi) the vice admiral that was considered for admiral position. If not her, then there are a whole lot of other vice admirals that are better than coby right now.