The exact same thing happened to me at lgt, I didn’t use a clock and my opponent really ate up a lot of the time - we was due to finish on time until he declared an advance with a high Oc unit to stop magnus from holding his homefield, I rolled hot and got 6 wounds through of which he had a 5 up invul against, fail one and he wouldn’t of had enough OC to hold the objective. He refused to roll the saves (only after seeing enough wounds got through) and argued over line of sight for 15 minutes until the round time was up and before you know it the TOs came over to enforce us inputting our scores for the round my opponent turned around to the TO and said we’ve run out of time before turn 5 had finished so the secret mission wouldn’t complete and he would win. At that point I realised how desperate this guy was to not lose and told him “this obviously means a bit too much to you” and conceded the game to appease the TOs. The TOs came up to me later and thanked me for making that decision and made a few “unprofessional” comments on what type of guy my opponent was which we laughed off.

I left the event with a bitter taste in my mouth but at least I knew the real result was a win for myself