@Phi: When you see the Hellebore, you will se the lance. Hemlocks are just a lance with solar wings, and the official models have some variation themselves, so everything shoud be fine as long as no escort gets bigger than the hellebore. The Hellebore is the largest escort.
While lances, Torpedoes and Bombers are often blocked by the field, every successsful block creates a blast marker, wich could damage the ship the following turn when it tries to move. Bombers can be efficient, because you don't have to form them in waves against eldar, since eldar ships have no turrent value, there is no benefit from forming waves. This means the ship has to make a separate roll for every bomber, which increases the chance to fail a save.
On the legion question: As Horizon said, do it. BTW: Marines increase your boarding chances. Boarding is often inferior to shooting, but it is a almost save way to get rid of an enemy ship without causing it to explode - and the player who holds the field at the end of the battle will earn additional renown points for each hulk on the field. So, maybe you could get some invested points back when the battle seems to run in your favor.
@Horizon: Yes, this weakness feels wrong. But if we dig a bit deeper:
At first, the corsair eldar are pirates. Their ships are not the "eldar golden age ships", they are refittet variants, refittet for their pirating needs: They are fast and lightweight constructions, able to outrun the ships of other races. They need to be fast, take what they can, and then they have to vanish. A battle on even terms is not where this ships should be used fluffwise, the corsairs are not an empire or a craftworld.
So, if we want to dig further, we should look at the ship classes originally constructed for partizipating in fleet battles: An these are the Wraithship and Dragonship class (Shadowhunters are craftworld patrol craft). At first, these ships have a higher armor rating of 5, which means that you need to roll 3 dice to get 1 hit, opposed to 2 dice for 1 hit - the "actual warships" have an 50% increase in weapon battery defense when compared to corsair vessels, and then there is the holofield-collum shift. Yes, they still can obtain critical hits easy, and there are no deflector shields, and they have a smaller mass than imperial ships - but they retain their mobility.
The next thing we have to take into account: Every wraithship or dragonship uses some kind of ordnance, some kind of weapon which could be used in long-range combat, and the mobility + speed to make sure they don't have to get into direct fire range.
Of course, you have to get in direct fire range ingame, because ordnance allone won't bring you victory. But this changes if we take the next thing into account:
When these ship classes where created, the eldar dominated the galaxy. Imagine a battle against a craftworld eldar force, but you are outnumbered. In this case, it would be impossible to touch the ships. This could also explain why their natural enemy of the old days (c'tan/Necrons) droped the ordnance weapons entirely in favor of 360° weaponry which could destroy lager amounts of ordnance.
if we look again at the holofield rules, we see that the field offer great protection against every weapon type which can be used in long-ranged combat: Ordnance and lances - the weapon types which would be most likely used in long-range engagement (because weapon batteries are getting inaccurate at long range).
Here, we can still ask: When they have been this superior fluffwise, why did they not just ad some deflector shields and turrent just in case? To protect against unforseen stuff?
Because of arrogance. The eldar are still arrogant of other species, even as a dying race, now try to imagine their arrogance against other species when they were the dominant race, in an age where "suns and stars die at their command" (i am not sure if this quote is correct, i am only remember the german quote at this moment). We are talking about a race which has no problem in pulling a sun into the webway if the need it, a race which obliterates whole civilisations because some farseer thinks this has to be done to prevent something unfortunate for their craftworld in the future, a race which kills humans without warning for setting their feet unknowingly on a maiden world...
Their arrogance has caused their fall by creating "she, who thirsts", and now they have to rely on technology which may be extremly advanced, but which was never intended to be used in fleet battles on even terms or in fleet battles at all. Its like opening a can with a knife. You can do it, but it would be more easy to use an opener instead.
Here we can ask: But why didn't they try to adapt?
They are trying - by inventing the ghostships. These were invented to address the "we are a dying race" problem and the "we have no ship class which is able to take hits from wide-spread-weaponry". Rulewise, ghostships loose the crittical hit weakness, but retain all defensive benefits.