My teammate and I were playing Lina (support) and Phantom Assassin together recently, facing a solo Darkseer in his offlane. This is around 3k MMR, so nothing too fancy. We handled it decently, but still had some problems nonetheless. To put it in a picture, we found ourselves often like this:
We are both low health from respectively trying to harass Darkseer out of lane, and last hitting / killing the Ion Shell creep (plus in this case a failed kill attempt - we still have some regen available).
It seems pretty tough to control the lane equilibrium vs a Darkseer early game, and many times the lane ended up under our own tower, making last hitting quite hard.
Some things we tried include trying to kill the Ion Shell creep when possible, harassing the Darkseer to low health, and just plainly letting it happen.
Some things I considered playing the Lina was in fact to just leave the lane (or at least go full-time stacking and pulling), as well as going all-out aggressive on Darkseer (it didn't work out for me though).
Note again this was a 3K public match, so things like carefully coordinated rotations weren't really an option.
Further note that I'm not looking for advice on counterpicking (I realize that there may be very specific counterpicks...): I want to know what to do when I end up in this situation. Preferably I'd get advice on how to deal with a Darkseer in his offlane in general (i.e. for the generic "ranged support + melee carry" situation), but if that would make the question unanswerable I'd settle for advice on doing this with Lina and Phantom Assassin.
So, the bottom line: during the early phase of the game, playing as ranged support plus melee carry in your safe lane, what can you do to counter a Darkseer spamming Ion Shell in the laning phase, if anything?