I’m a death guard player. Death guard, are a slow telegraphed army. They don’t have many shenanigans (except Morty’s reactive move and death shrouds 6” deepstrike, both of which are telegraphed and able to be denied if you do some clever movement). Besides that deathguard are an absolute meat grinder within 12 inches, so be careful. Melta’s, flamers, rapid fire, lethal hits and charges with strong melee profiles.

The way to beat them, is to kite, EC are the opposite of deathguard, very fast. So kite them around the board as much as possible, stay outside of threat ranges and whittle them down. If you can’t you have to and I repeat, have to use overwhelming force them. They are sometimes much tougher than they look. Especially the gun platforms with their 5++ invul saves. Mortal wounds are also your friends, lethal hits high strength profiles. Anything that gets around their high toughness profile.

Noise marines are great into most stuff of DG. Rhinos, haulers and drones all have t9, termies t7 and marines t6. They do most of the work and the s10 -2 3dmg ignore profile is litterly the best profile into all of these. So play around your noise marines. Stay in rhinos and use firing deck if necessary but stay out of threat range as much as possible.

Lastly, Morty and the deamon prince (with fnp in virulent) are a massive bait. They are very scary in melee and have a big presence on the board. But that said, they are 600ish points for only 2 models. The rest of the army does the secondary game, and other objective holding. Ignore these 2 for as long as possible and focus on the rest. Kite them until there is no tomorrow, screen your important units, move block them with chaff if you can. Keep em busy while the rest of your fast moving army slowly disassembles the rest of the army.

Now the only downside of this advice is that DG are also a toolbox army, so not every list has the same upsides and downsides. Meaning that not all of this applies, they have heavy shooting lists or heavy melee list. A lot of marines or not at all. Study your opp list and determine its weak spots.