should there actually be "indestructible" weapons?

[Varyon]

Gear doesn't have to follow the same rules as characters, so indestructible gear is acceptable. And, of course, characters can have indestructible traits. Damage Resistance cannot be reduced except for by corrosion attacks, and I'm pretty certain an application of Cosmic, Added Utility +50% will get rid of that. The last level of increased resilience for Natural Weapons is Indestructible. Injury Tolerance: Unbreakable Bones is a thing, as is Unkillable 2 (where you are described as leaving behind an indestructible skeleton or similar). A melee Innate Attack that is described as a summonable sword (of matter, energy, or whatever) is similarly indestructible (in fact, if you built a force sword as an Ally, the blade part of the force sword would be an Innate Attack, not part of the Ally's body).

Now, certainly it would make sense for something of sufficient power to overwhelm a force sword - after all, the force fields it's made out of are explicitly destructible (being rapidly-regenerating, semi-ablative DR). The issue, as others have noted, is that the amount of power you need behind the attack means that the collateral damage from it impacting the force-sword's blade is likely to destroy the hilt anyway (and probably the wielder, for that matter). As for breaking it by weight rather than damage, unless the character has godlike ST yet is wielding a force sword made for mere mortals, anything heavy enough to break the force field is going to be too heavy for the character to Parry.

If you really want to know how much force-field DR the force sword's blade represents, here's how I'd do it. Find the smallest device that generates a force screen of an explicitly-described size (one from which you can determine its area - force screens are essentially planar and lack meaningful depth). Divide the weight of the force sword's hilt by the weight of this device, then multiply the DR the device grants by the resulting value. The result here is how much DR the force sword hilt would provide if it generated a force screen the same size as the generator does - but of course a force blade is markedly smaller. Personally, I envision the "blade" of a force sword as being something like a cylinder that's roughly as wide as a pencil (1/4"). A 1-yard-long cylinder with a radius of 1/8" has a surface area of roughly 28.37 square inches, or roughly 0.2 square feet, or roughly 0.022 square yards. So, divide the actual area by the above, then multiply the calculated DR by this value. Note this assumes the power supply scales with the device. Because you've crossed from the higher-efficiency power cells (size E+ IIRC) for the force screen to the lower-efficiency power cells for the force sword (size C), divide the result by 2. Additionally, if the weight and time of the power supply didn't scale linearly, you need to adjust for this - divide weight of power supply by weight of the hilt/generator in each case, multiply each by how long they last, then divide the result for the hilt by the result for the generator, and finally multiply calculated DR by this value. In other words:

(Weight of hilt)/(Weight of generator)*(Generator DR)*(Force screen area)/(Force blade surface area)*((Hilt power cell weight)*(Hilt time)/(Hilt Weight))/((Generator power cell weight)*(Generator time)/(Generator Weight))/2 = (Force blade DR)

This simplifies to:

[(Generator DR)*(Generator Area)*(Hilt power cell weight)*(Hilt power cell time)]
[2*(Force blade surface area)*(Generator power cell weight)*(Generator time)]

Of course, that tells us how much DR it would have if you made a force sword that functioned as a bludgeon, not one that burns through stuff. You're going to lose some efficiency to burning through things. Additionally, being so small may make the force field generator less efficient (beyond simply being required to use less-efficient power cells). This part I'll leave up to you.