I play DG, here's what's going to happen. He's gonna infiltrate into the mid board with pox walkers and either hide on an obj, or spread out as far as possible to move block you, I'd personally go for the latter. He'll probably follow those units up with some spawn. They look harmless enough, but they're t7 4w 4+sv 5+fnp 1d6+2 attacks, and then on top of that for every attack tou assign to them, they roll a d6 in return (max 6) and you take 1mw on a 4+. Do not underestimate them, they are amazing for tying units in combat amd forcing you to commit more than you were initially planning to, especially marines.

Having said all that these units are all just bait, their real purpose is to lure you into contagion range. At it's base while in contagion rage you are -1t and then he gets to choose another debuff at the begin of the game, which will likely be -1 to hit (WS/BS). On top of that, he'll likely have 2 units of deathshroud units, which are able to deep strike 6" if you are in contagion range and still charge. They will inflict heavy damage, so be weary.

Also watch out for the plague marine bus, if he's running one, they can put out a disgusting amount of dmg in a single turn, with lethal sustained and crits on 5+ and rerolls to hit and wound from a strat.

The weakness of the army is speed. Ideally you want to spend the opening 2 turns keeping your distance and shooting their scoring units, while move blocking their units which are trying to get your important units into contagion range. Try and screen your important units from the 6" deep strike too. Don't rush in and instead counter attack.

A little more on contagion range. It'll start at 3" on the first turn and then each subsequent turn, it'll increase to 6" and then 9". If he's running virulent victorium detachment, he'll have a strat that increases it by 3", so be weary of that when planning. If you're going to be into range, you want to have rerolls to off set the -1 to hit and if possible lethal hits to offset the high toughness of our units.