The Relentless ability for the Trandoshans reads: "When you delcare an attack targeting a figure within 3 spaces, that figure suffers 1 strain."

My question is - if that strain causes the figure to be defeated and withdraw, can you still perform the rest of the attack? This is relevant for the Elite Trandoshans because they could become focused by that attack.

I saw in another discussion that the official ruling about Relentless is that its damag is not considered to be "part of the attack," but I don't know if that helps clarify this at all.

If the target is defeated and this causes the figure to be removed, become Incapacitated, or forced to Withdraw before you roll dice, you cannot "perform the rest of the attack".

Note that if a healthy hero is defeated by relentless, the hero becomes wounded (flip to the wounded side, perform wounded hero steps as per "Wounded", RRG, Page 25) and then you continue the attack.

You can't attack a target that is no longer on the map. (This also applies to other abilities that require you to attack the same target or target figure.)

Even if you could, blast, cleave and conditions require that the target suffered damage from the attack (step 7), so the Trandoshan Hunter could not become focused in any case.

So, I'm assuming also that this would still count as an attack, consuming an action and triggering "after an attack resolves" effects?

It would still count as an attack, consume an action, and trigger **some** "after an attack resolves" effects, depending on the wording of the effect.

(I say **some** because many of these effects rely on certain triggers happening during the attack, which in the above case, may not happen).

Yep. It would count as an attack because to be able to declare target (step 1 of the attack) you have spent an action (or ability such as performing an attack from Executive Order or Sustained Fire) to perform an attack. Even if the rest of the attack can no longer be resolved, you have performed an attack.

And if the trandoshan declares the attack and is focused before declaring, does the mini lose the focus state?

Now you're asking a difficult question.

Focused: ... After you resolve an attack or attribute test, you must discard this card.

If Relentless defeats the target, you spent an action (or ability) to perform an attack, you declared target, but the attack was not actually resolved and thus focused is not discarded. (No after attack resolves abilities can be performed either.)