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How do I counter this with Marines?

Farseer on bike
Warlock on bike

2 vypers with shuriken cannons

3 squads of 5 scatbikes

Wraithknight with heavy cannons and 2 scat lasers.

This is stupid fast and I find all I do is react.

GravCents with Tiggy and SkyHammer (probably with grav and a CombiMelta or something for flexibility).

Null deploy, then drop down and ace the Wraith Knight with all the Grav and take out 1-2 units of bikes with the assault marines (flamers, I imagine, and maybe a power-sword sergeant).

But, if I only had one word: Grav.

For this particular list, I vote for a mallet.

That's where I've been leaning!!

DCannon had preety much covered the solution, Grav cannon Skyhammer or Droppod Grav Cents are probably the only answer we have to the OP Eldar army. It is also good choice to add an Xeno Inqusitor with Servo Skulls to land your assault marines more precisely to ensure a immediate charge after landing.
One thing to beaware is that once the Eldar player knows what you have, he may probably reserve his WK to avoid your alpha strike. In such a case you need to table him before his WK come in. Given his semi MSU nature of the Scatterbikes, split fire or the ability to combat squad is important, that ypu can avoid overkill one squad and the others are unharmed. So either max out the Devastator Squad in Skyhammer, or let your Inquisitor take the tome of vertic (the BIG I's relic against xenos), as it is the one grant spilt fire against Eldar.

My problem is that he pops out and fires and then uses battle focus to pop back out of LOS.

Not after the mallet, he doesn't.

hisshers wrote:
My problem is that he pops out and fires and then uses battle focus to pop back out of LOS.

You should be dropping close enough that he either is turtling in a corner or getting murdered.

Could you post an example of a list you have taken?

There are lots and lots of different ways SM can counter this but it is entirely dependent on the chapter and play style.

Thunderfire cannons, grav on bikes, flamers.
May well just be my opponents, but scat-bikes have not been scary for my marines at all. Wraithknights go down to focused grav pretty well.

Be aggressive, don't let them control the board, don't be afraid to get up in their face (and lose a few marines doing so). Pods, scouts, speeders; anything to shut down his mobility early on, then torch 'em.