So I finished the Kraken fight yesterday with a friend. When you have to exit the arena because it’s getting hot in there, only one player can make it. The one a bit further behind gets blown up. Every time. My god, has anyone tested this game before throwing it on the market?
Anyways, the first time this happened, I managed to trigger the cutscene and achievement despite my friend being dead. It would take us into an empty space, white for me, black for him. Very weird. Cutscene sounds and voices would play, but not the video. Guess our characters get put into Limbo whenever a cutscene plays. But it wouldn’t progress. We were stuck.
Upon checkpoint reload, we were back to final Kraken stage. Beat it, like, fifteen more times trying to get out the tunnel closer together. Last time was Del kissing Kait’s butt on the way out, it still blew poor Del up. I deem this impossible to do. The game counts act III completed, however, so I hope the achievement will pop after act IV. But even if it does, this is a massive and worst, OBVIOUS oversight.
Man, TC, just playtest your crap, alright? Next gears, don’t release until you, yourself, have beaten it on highest difficulty, solo and co-op. Problem solved.
Me and my friend are having the same issue right now we are playing the campaign co-op on inconceivable & are finding it impossible to make it out of the tunnel ? Is there a work around for this or a patch or something . How do you botch the campaign this much. This is not the only place in the game where this sort of thing happens. Come on Coalition letting us down big time. 
Update: we made it out of the tunnel by literally running side by side and although we both survived somehow but the game crashed and did not give us the progress after the cutscene.
If you only have two players, one workaround is to have the second player play as Jack, as I believe he is invincible during escape scenes.
I played coop once in that section and using the Stim ability of Jack to prevent damage as there is stuff exploding behind you which could catch the second player seems to do the trick.
Was it on inconceivable? I tried having my friend play as jack, We tried using stim; tried using cloak to run faster. We switched characters (Kait and de). Then, by the graces of god we both made it out alive by basically walking and running exactly side by side and dodging explosions and then the game crashed. His screen went black, but the audio from the cutscene still played, but then it reset us to the checkpoint. I think we got credit for it, but that is TBD until we finish it.
We do were playing it on Experienced coop(Insane eventually got too bothersome to carry on even if we were managing it), so it’s a very likely possibility my experiences don’t work for Insane and Inconceivable and that it is very much a bug. Some clips would help, rather than just text.
When we finished the campaign we got the achievement for completing it on inconceivable. I’m not sure at what point it counted our attempts as successful. As I mentioned, we did make it out alive one time, only for it to reset us to the same checkpoint. At the point of exhaustion, I started a new campaign at the beginning of Act 4, because that was the next chapter. I wanted to preserve our previous save point so we didn’t have to go through the whole chapter leading up to Kraken in the event our progress was not counted, Bottomline - If your getting stuck trying to escape through the tunnel after the Kraken fight check your achievement status for completing the campaign on inconceivable and if it is at 86%+ then you are OK to proceed to Act 4. If not, then god bless.
For anyone who comes here after playing GOW5 on inconceivable coop:
We figured out if you walk slowly side by side, youll get past this bug.
Even with doing that, you may die and just barely get the cutscene.
Its checking for the player furthest down the tunnel, and blowing up behind them. I do not believe for a second it was tested coop on this difficulty.
A few years late but I just encountered this issue with a friend. We did the following: The lagging player (Del in our case) does not try to run through the escape tunnel. Instead, the leading player (Kait) high tails it out while the lagging player stays in the arena. If the lagging player stands in the right spot and utilizes stim, the leading player can escape and trigger the cutscene before the lagging player dies. Only took us two tries.