[DonRotolo]

Remember that in battlebots pneumatics of 1000 PSI are not unheard of. So power will be plentiful.

I also don't like the idea, since pincers are vulnerable, but nonetheless:

One issue with pleumatics is that the darned actuator cylinders are so long: Retracted they are about 4 or 5 inches longer than the 'throw' of the piston, so for a 6" actuation you need about a foot.

Two cylinders will give you twice the grip, as opposed to a single cylinder acting on both arms equally. You can also operate them independently, whacking outwards if that's advantageous at the moment. Sorta like a smack upside the head.

Hydraulics will be just too slow, if the intent is to 'whack' the opponent. If you intend to grab and crush the opponent, hydraulics have the advantage, in that the fluid is not compressible. Think jaws of life.

In either case, both ends of the cylinder need to be pinned with one degree of freedom, since the angles to the chassis and the pincer will vary.

If you push with 1 ton at the 6" end, your 18" end will see 1/3 ton. Do what you can to reduce the long end and increase the short end, to get more strength. But if speed is the weapon (think fast hammer) then inertia will do the work, and your 6"/18" would probably be good. The business end tips need to be hard and heavy, perhaps with spikes pointing inwards to inflict maximum damage and get a good grip. But gripping a massive spinner may toss your robot pretty hard, bend the arms, or worse. But imagine if you could dig in and hang on! Wow, that'd stop that spinner real good.

massive axles, 2" diameter by 1/2" wall aluminum might be strong enough. Roller bearings for sure, top & bottom. Larger diameter can handle a lot more stress. a 1/2" bolt will tear right out.

Your arms need to be strong on the side to side direction, but not so much up and down. But guard against the sledgehammer bots, those arms are so very exposed.

Weight class and sketches will help.