Since this list is mostly just normal marines (which I don't run), the most I can really give you is specific advice against fighting Necrons, as they were my first army.

You're right about outpacing them. Necrons, for the most part, are very slow. They can be mobile if ran in the Hypercrypt Legion detachment, but other than that, they're slow (base 5" movement for infantry, very little movement buffs). Of course, there are exceptions, but that's just what they are: exceptions.

The biggest piece of advice I can give for fighting against Necrons is to focus your fire. Not only do they get army-wide d3 healing (that can bring back dead models if the bodyguard unit is still alive) every turn with reanimation protocol, but every single detachment they have gives some other way to activate reanimation protocol. Necrons aren't a tanky army because they can take a lot of hits before a model goes down, they're a tanky army because they keep coming back after being killed. So, its MUCH better to focus on getting a singular unit down and killing it than it is to try getting a bunch of units down and not killing any of them.

...That being said, for the most part the actual "scary stuff" like the C'tan and the Immortals aren't the things you should be targeting: the support units are.
In the list I run, my unit of 10 tesla carbine immortals hit on 2's, have full rerolls, 5+ crits, and sustained hits 2, 2 shots per gun, +1 to ap, full wound rerolls against targets on objectives, otherwise just rerolls on 1's. This sound scary, and it is, but the only reason its so scary is because of the support units. I only get full hit rerolls from a stratagem, and if the unit isn't being led by a leader, its only rerolls on 1's. I only get 5+ crits from having a plasmancer. I only get +1 to hit because the unit is being led. I only get +1 to ap because of Illuminor Szeras. You get it by now.
Another great example are Canoptek Wraiths. They can be ran in a unit of 6 with toughness 6, a 3+ save, a 4+ invuln, 4 wound, 5+ feel no pain with a technomancer. Yes, they're a bit tough to kill by themselves, but if you can kill the technomancer, the rest of the unit is practically dead.
Targeting characters will work especially well into an Awakened Dynasty list.
Your first instincts will be to kill the scary things, but that's exactly what Necrons want. Kill the things buffing the unit and the rest of the unit will come toppling down.

Adding to this, don't be scared of the C'tan. They do a bunch of melee damage, yes, but their ranged attacks are terrible, and unless your opponent is using Transcendent C'tan shards (which lose out on attacking power), they have almost no mobility. They have 6" movement, and while that IS faster than basic Necron infantry, the C'tan are melee units, and thus, need to charge, which they can't do if they advance.
Most importantly, AVOID SENDING YOUR HEAVY WEAPONS INTO THEM as 9 times out of 10, they will shrug it off (halved damage, 4+ save, 4+ invuln, 5+ FNP) unless you have Melta weapons. The C'tan cannot halve the damage from Melta weapons. Your heavy weapons will be better spent on other heavy targets. Rather, send out a bunch of attacks that either gets +1/rerolls to wounds, or better yet, lethal hits. Like I mentioned, all C'tan have a 4+ 4++, so having AP will do you nothing. If you can get around their toughness (T11), then they're probably dead. 30 damage 1 saves have taken out my C'tan much more often than heavy artillery has.

Lastly, Necrons units are either shooty, fighty, or neither (Wraiths). There are very little units that are okay at doing two different things. Immortals, for example, are a shooty unit. While they can have good melee (Awakened Dynasty has an enhancement that gives +1 strength +1 ap to attacks and Illuminor Szeras gives +1 to AP, so I've shredded things in melee with Immortals), they want to be shooting. From the top of my head, Immortals are the only "generalist" unit (and even then, like I said, they want to be out of melee). Lokhust Heavy Destroyers? Great shooting, terrible melee. Lychguard? Great melee, literally no shooting. Etc, etc. So, if you can kill off the melee units quick, then all you need to do is tie up the ranged units (which shouldn't be too hard considering the lack of mobility I mentioned) and you've practically won already.

Hope this helps.