What I'm talking about is during end-game in a fairly balanced game once everyone is packing artifacts and level 20, Oak is rushing into your towers, forts and even citadel and wailing on them till you or the building kills him then using last stand to get a guaranteed kill on a tower or fort with the increased damage, and is definately going to be taking up to 20-30k out of your citadel (in my case he was taking about half of it's health with maxxed building health upgrade). While to some of you this may sound like free gold, keep in mind that by this point in the game both teams citadels have almost every single upgrade, and everyone is using a full inventory of artifacts. Pushing up while he was respawning wasn't a viable plan because his teammates would nab a flag near our citadel and he'd teleport back down there wailing on the citadel and we'd have to port back.

Needless to say we lost after 5 or 6 of his suicide runs. ( It was still a great match, and the most evenly matched game I've had up until the last part where the suicide began )

Someone suggested healing the citadel back up with universal gadgets, now I was doing exactly this but those things have 10 secs of cooldown, which if only one of you is doing it means it's gonna take you about two minutes to heal the citadel back up while Oaks teammates push up even more, and Oak is definately going to be back before those 2 minutes are up. Then they suggested I get all 3 teammates healing it, this would of went even worse because who is then going to hold off Oaks two teammates from pushing up to the citadel as well?

Could someone suggest any ways to stop this?

Because apart from fighting in an uneven game where your team has a huge advantage the entire way I can't really see any counters in a long game versus Oak doing this.