Another confirmed #deca cap here! Lovely trophy to have. v2.5 vanilla game.
I spent about an hour and a half in total trying to cap this bugger, and at one point I almost thought it was uncappable.
Anyway, heres how i did it:
Team 1 - 20x 2* fighting marines for the initial boarding operation. They did fine and at least 7 of them make it to the core every reload attempt, and complete the hack ok.
Team 2 - 20x 3 and 4* marines for the second wave.
Team 3 (backup) - 60x 1 and 2* marines aboard 6 TP's that I also sent in for reinforcement in case Team 1 couldnt make it. Good job i had these as i used 10 of these for assisting Team 2!
So the problem i was having that was frustrating was that when waiting for Team 1 to hack before sending in Team 2, the hack would complete before Team 2 had cut into the hull. Team 2 would get pushed into space, and the #deca wouldnt have any marines on board.
So i looked at the timings. I waited until Team 1 had cleared deck 4 and begun assaulting deck 5. I managed to time it so that i guess Team 1 were half way through that deck and then i launched Team 2 from the pods, that way, Team 2 were cutting BEFORE Team 1 started the hack. This seemed to do it for me, although i only ever seemed to get 12 of Team 2 on board and fighting. These 12, no matter how many reloads i did could never get past deck 3 and always fail. ( I was pulling my hair out at this point

)
I had Team 3 as backup, and using some swift keyboard punching, as soon as Team 2 launched, i transported another squad of 10 from Team 3 onto my Cobra, and immediately launched them. So as the 12 from Team 1 were through the hull, this other squad of 10 were cutting in.
Well that seemed to do the trick, as this other 10 joined Team 2, giving me 20 marines clearing deck 1. I was saving frantically at this point, with saves at each deck. Eventually after a few more reloads, 5 marines from the 2nd and 3rd teams got to the core and hacked it! Adding one #deca to my property list!
\o/
I hope the above helps others that may have had troubles with their timings.
Edit - I lost quite a bit of rep with the Terrans when Team 2 started their assault, so be warned.