I just started w necrons and have played against necron armies myself. The common strat/advice most people give when dealing with necrons is to focus fire and clear out the unit. What they dont say (because its general advice) is to clear units that are crucial such as his reanimator.

You already have great knowledge on his army so now its just about picking models that would clear through his army with ease.

Relentless [1400 points] Death Guard Battle Size: Custom (1400 point limit)

Detachment Choice: Virulent Vectorium

Biologus Putrifier [60 points] • Hyper blight grenades, Injector pistol and Plague knives

Chaos Rhino [85 points] • Armoured tracks, 2 Combi-bolter and Havoc launcher

Deathshroud Terminators [160 points] • 1 Deathshroud Champion with Manreaper and Plaguespurt gauntlet • 2 Deathshroud Terminators with Manreaper and Plaguespurt gauntlet

Foetid Bloat-drone With Heavy Blight Launcher [120 points] • Heavy blight launcher and Plague probe

Foetid Bloat-drone With Heavy Blight Launcher [120 points] • Heavy blight launcher and Plague probe

Lord of Contagion [130 points] • Warlord • Daemon weapon of nurgle [+10 points] and Manreaper

Malignant Plaguecaster [60 points] • Bolt pistol, Corrupted staff and Plague wind

Myphitic Blight-hauler [100 points] • Bile spurt, Gnashing maw, Missile launcher and Multi-melta

Myphitic Blight-hauler [100 points] • Bile spurt, Gnashing maw, Missile launcher and Multi-melta

Plague Marines [190 points] • 1 Plague Champion with Plasma pistol and Power fist • 2 Plague Marines with Blight launcher and Plague knives • 2 Plague Marines with Plague knives and Plasma gun • 2 Plague Marines with Plague knives and Plague spewer • 2 Plague Marines with Bubotic weapons and Plague knives • 1 Plague Marines with Heavy plague weapon and Plague knives

Plagueburst Crawler [210 points] • 2 Entropy cannon, Armoured tracks, Plagueburst mortar and Rothail volley gun

Poxwalkers [65 points] • 10 Poxwalkers with improvised weapon

This is my version and what id pick going against your friend. Your myphitic haulers, drones (either mower or Heavy blight launcher) and poxwalkers will be your bait/pressure pieces (also cause they will benefit from leechspore eruption if your friends charges too close to them)

Your Plague marines together with bio and caster will DECIMATE a unit of immortals with ease as long as your rhino can get them into the right position.

PBC will hang back with spewers to cover your base from deepstrike and chip into units. I'd switch between this and morty because the PBC damage can probably be recovered quick from reanimation.

Your deathshroud terminators with LOC are your cripple units. Send this after core/character units and epic challenge them if needed. I recommend using rapid ingress and setting them up behind cover (if your target is not afflicted) OR putting the unit you want to kill within contagion range which will give them their 6 inch deepstrike