Fly around him, and get yourself just outside the border. He’ll try to kite away from you, and move back into the arena. Alternatively, prevent him from getting that far by establishing a tight orbit around him early, before he can move far from the conduit.
Its a freagin bug that they just won’t fix. There are a few more. Happens to me all the time. 3 times today alone, 2x drifter BS on different sites, and 1x that Dominix looking drone BS, it was behind the gate as I was burning at it and I guess maybe it bounced off and headed in opposite direction at really fast speed with MWD on ? Reminded me of mission Machariels, then it went outside the boundaries and sat maybe 20 km out behind instance wall shooting.
So what I do now when I see them moving off is to immediately burn for that BS while fighting anything else along the way. Once I get the low crap (drones frigs cruisers) thinned out enough so I can face tank it, put my highest DPS drones on the BS. I kinda circle back and forth by the boundary, watch the drones, when their shields get to about 1/3 I recall them and swap them out. Rinse and repeat until the battleship is dead.
Forget sentries, you’ll lose so much DPS and tracking you’ll die to everything else, unless you’re in an Ishtar and have enough space to have spare flights of drones.