You can alter critical hit ratio in the following ways:
Ability: Pokemon with the ability Super Luck have heightened critical hit ratios. It should also be noted that while Sniper does not raise the chance for a CH, it does make your attacks even stronger (triple) when it happens.
>Using the move Focus Energy, as you stated, will raise CH chance.
>A Pokemon holding a Lansat Berry, Razor Claw, Scope Lens, Lucky Punch (Chansey Only), or a Stick (Farfetch'd Only). The last two raise the Critical hit chance by two stages.
>Using the moves Aeroblast, Air Cuter, Attack order, Crabhammer, Cross Chop, Cross Poison, Drill Run, Leaf Blade, Night Slash, Psycho Cut, Razor Leaf, Shadow Claw, Slash, Spacial Rend, or Stone edge will raise your odds.
>Using the move Frost Breath or Storm throw will always result in a critical hit, barring of course, some negating ability (.
>Using the item Dire Hit will also increase it.
A critical hit now multiplies damage by only 1.5 rather than 2. The Sniper Ability still multiplies damage from a critical hit by 1.5, resulting in 2.25 times the usual damage. In addition, the probabilities for each critical hit stage have been modified, greatly increasing the likelihood of a critical hit for stages +2 and above:
+0 Stage = 6.25% CH
+1 Stage = 12.5% CH
+2 Stage = 50% CH
+3 and above Stage = 100% CH
The highest stage you can get in Gen VI is an attack has a high critical-hit chance (12.5%) + Farfetch'd holding Stick or Chansey holding Lucky Punch (50%) + Dire Hit (12.5%) + Focus Energy (50%).
Hope this helps.