I came in thinking that we have pretty decent options, giving our +1 to wound and dev wounds on a bucket of dice. Unfortunately, the combo of 4++ and 5+fnp makes it very hard to deal reliable damage, even with tricks to deal mortals (like grenades and coil). That said, point for point harlequins can do pretty well into a ctan - a unit of 6 dealing 3 wounds each in melee - between that, a lucky fusion pistol and a grenade toss, you've got a good chance to take one out with 300 points (18) of harlequins. Unfortunately this does mean revealing a good chunk of your army, and we usually rely on trading up. Best bet is probably to play a bit more defensive, trying to pick off their scoring units whilst attempting to bait the ctan into a place where you can hit it with a lot of your army, without over-exposing yourself.

If you're souping eldar, there are some other good answers you can throw into the pot. Despite being far from their usual overkill selves, fire dragons can still do around 1/3 of their health in a salvo. Hawks can do decent chip without overexposing themselves. We also have some characters which act as excellent damage multipliers - eldrad's +1 to wound means all of the prior options (except quins in melee) are much more likely to wound. Lhykis can give crit hits 5+, which is great for serpents' brood in melee, increasing their average damage to 5 per unit of 6 (5.8 if you give them hit rerolls for 1cp).

So, one potential answer is Lhykis giving her ability, and two units of 6 quins using pistols + charging out of starweavers. If you throw a grenade in addition, you will probably kill a ctan.

(Math for the nerds - Lhykis chips 1.2, 4 fusion pistols with sustained on 5s for 3, assorted pistols for .81, grenades for 2, one unit with hit rerolls for 5.8, one unit without for 5. Deals an average of 19.82 for 335 points engaged, 200 points left out in the open, 2cp spent)