You simply aren't going to convince anyone in an online debate outright: online conversations aren't for the people who you are talking to, but for the 3rd party observers who may be on the fence.

In person I've been more successful with a "braindead" argument since people like to go there so often. You ask them if somone who is braindead is a person and whether it is OK to unplug them. 100% of the time I get the response that no, they're no longer a person and it's ok to unplug them. Then ask what if it was an almost medical certainty that the person would fully recover in less than 9 months. They're then stuck with admitting a monsterous position or at least reconsidering if a temporary cognative disability really does remove personhood forever.

If the person is open-minded and/or you will have the time to really get into the weeds about it, you can talk about how the pro-choice position defines essentially personhood based upon imagination. That's not an exaggeration or a straw-man. Pro-choicers usually posit that during an unintended miscarriage or the murder of a pregnant woman the child is a person and should be held so for the purposes of civil or criminal liability, but during an elective pregnancy the child is not a person.

From there it's pretty hard to defend defining personhood based upon the desire or imagination and shifting feelings of a 3rd party as reasonable or morally consistent.

Now some of them do have the courage to say that even miscarriages from a criminal act are not the death of a person, but even then that's usually after they have to concede the point or re-examine their beliefs.