How can I deal with a -speed + def LA Lyn?

Keep her Bond skill and give her a Spd refined Owl Tome. The refinement won't quite cover her bane, but comes pretty close. She's a great candidate for mixed tank, as her speed is still quite respectable, and as an armored character, she's gonna stick by her allies a lot. Even with only one ally beside her, with Bond/owl she gains +7atk/spd, and +2 def/res. This is before considering any sort of spur/drive/ward/goad the ally might have.

You can also add a Spd+ sacred seal. True, her IVs are not the best, but they are very workable. She got blessed as being a unit who can fairly easily recover from a poor IV spread.

You make her a defensive tank, that's what.

Make her march beside her armor bros and get those defensive buffs. Let her be doubled even, so she can proc Glacies faster. Give her Blarowl, if you want a stat increase when she's beside her armor friends.

Edit: Make sure to refine her tome for even more def. If you ever get an extra W!Tharja and some CC fodder, you can run W!Tharja's build on this Lyn better than Tharja herself does, since Lyn is blue.

Who needs speed when you've got high defenses and you've got Guard?

-SPD isn't the end of the world for her, as she is still pretty fast for an armor. (Compare her 33 Speed with Black Knight and Amelia, who have 34 at neutral).

You can turn her into a ranged mixed tank that is still decently fast, especially if you keep her default A and inherit a Blarowl+ with SPD refine, though you could opt for one of the defensive refines instead. Just make sure she is sticking together with those armored allies with buffs (Hone/Goad to increase offensive presence, Fortify/Ward for defensive, or some combination of all the above), and she will still works spectacularly. You can also add the Spd+3 seal if you really want, or stick with DD3 or QR3.

In my opinion, -SPD isn't even her worst bane she could have (that would be -ATK), and she still performs amazingly well with the lowered speed.