As long as I pay multiple resources, am I able to increase this card's ATK value more than once? TIA.

As it doesn't have the "limit once per phase" or other restrictions (like Nazdreg’s Flash Gitz) if you have the resources this guy can get scary when enough resources are present.

Just remember that each resource is a separate use of the Action, so your opponent will have the chance to take their own action between each use/resource you spend.

(I figure you already knew this, but you'd be surprised how common an error it is for people to think they can trigger the effect once by paying 3-for-6 instead of having to pay 1-for-2 three separate times.)

If this becomes a thing do you think FFG will make a limit once per attack ruling or something?

I thought that they were trying to avoid situations where warlords can be "one shotted" without being able to do anything about it. I.e. if the DE player has initiative (and both players send their warlords there) then your warlord can be taken out before you have a chance to retreat.

Especially with DE having Raid, even if you plan the math, they can switch it in a heartbeat and suddenly be swinging for 10 - which pretty much kills all warlords no matter how many shields you're holding.

Indomitable, Glorious Intervention etc can respond to this, but some factions have nothing!!

Let's see by faction:

• Space Marines have Indomitable and Iron Halo
• Astra Militarum have Glorious Intervention, Markis, Suppressive Fire and Preemptive Barrage
• Orks have Kustom Field Generator
• Chaos have Tzeentch's Firestorm
• Dark Eldar have Archon's Terror
• Eldar have Foresight and Eldorath's ability (and generally win command, so the DE player is unlikely to have the resources to pull that off)
• Tau have Fire Warrior Elite

So each faction has something to foil that plan. And its allies have something anyway.

On the other hand it is a high risk strategy, you have to invest a lot of resources to get it up to an attack value that can one shot. Once all those resources are spent, there is nothing to stop the opposing player pre-empting the actual attack with a combat action to kill or otherwise avoid the attack from happening.

There are other options such as ranged attacks or just making sure your warlord ends up not on that planet.

It is nasty if you don't expect it, but I'm not sure it is game breaking.