Dies Irae

First we need to understand what things do.

There are four damage types: Kinetic, Energy, Internal, Elemental. The first two are mitigated by armor, the latter two are not.

There are also four attack types: Melee, Ranged, Force, Tech. You can consider the first two to be “weapon attacks” and the second two to be “spells”  It’s not always clear which is which by the tooltip, but when you use them weapon attacks will show up as white numbers, spells as yellow.

Attack type and damage type have no correlation, you can have any combination of the two sets (though not all combination exist in game). The first two are subject to defense (avoidance) and shield (partial mitigation), the latter two are subject to resistance. Resistances (which are just avoidance for Force and Tech attacks) are present as a mechanic, but very very few effects in the game grant them, so they can essentially be ignored.  Weapon damage, unless otherwise stated, is kinetic or energy damage.

Thus you can very easily have an attack that bypasses armor yet can be dodged or shielded (an internal melee attack, for example), or an attack that bypasses defense and shield yet is armor mitigated (an energy force attack, for example).

Here’s a handy chart.

From this it should be clear that spells that do internal/elemental damage will ignore most of the defenses of tanks.

Weapon damage attack start with a base 90% chance to hit and spells start with 100% chance.  According to the description accuracy improves this for both and defense reduces it.  This is slightly misleading because you’d think that someone with a high defense of say 30% could avoid many spells, but that is not true, in PvP spells always hit.

What this means is that if you can minimize the number of weapon attacks in your rotation the affect of accuracy is next to nil.  The same way someone who uses only instant attacks is unaffected by alacrity.  Also using only spells against tanks eliminates the effect of their shield generator.

PVP gear is covered with accuracy, however it is usually in the enhancement slot.   By replacing these enhancements with other more useful stats you can greatly increase  your effectiveness.

What stats to use?

Things that increase your percent chance to do something have a cap.

Stat Caps:
Surge: 50%
Crit: 30%
Alacrity: 30%
Shield: 50%
Absorption: 50%
Expertise: 20%

This is for your stat rating.   For example, the max crit rating you can have is 30%, but you have a 5% base chance to crit.  If you have talents that increase crit chance by 9% and  the smuggler/agent buff that gives an additional 5% you can have 49% crit chance.

Your primary stats (aim/strength/cunning/willpower) and endurance do not have a cap.   Neither does the secondary stat of power.

Because the variety of high level mods is extremely limited I find myself stacking things like crit and surge to near cap without really trying, simply because there is nothing else available.  That  is why, when in doubt, I always go for power as a secondary stat.