Looking at our Partner Day schedule, I think it would be best if you left right after the campus tour, at 12:30. You can either use public transportation or take a taxi to get from Ajou University to JW Marriott in Dongdaemun Sq.
If you take public transport, you would need to take an express bus from Ajou to Sadang Station and transfer to subway line 4 to get to Dondaemun Station (~1hr30min). If you take a taxi, it will cost about 50 USD and take about an hour.