creature ability DCs are not just arbitrarily assigned, they are generally (DC 10 + 1/2 creature HD + Charisma modifier). so your paralyzing gaze ability would have a DC of 10 + 5 - 2 = 13. ways of increasing this would be changing the ability score it is based on (constitution is another common one) or increasing its hit dice or the ability score it is based on. there is also an "ability focus" feat which gives +2 to the DC.

your "inhuman grip" ability should just give the creature a +4 or so (this number you can decide, but pick a number, don't just say "give or take") bonus to grappling checks.

if you want it to have AC 20, you have to say why. it has 10 base (every creature does), +3 from Dexterity. that means you'd want to give it +7 natural armor. if it's the dodgey type, instead of giving it natural armor, you could increase its Dexterity.