Shadowheart stats got changed in a recent patch so she has 15 Dex now instead of Str.
Shadowheart is still a great tank and can safely be in melee range. No need to melee attack with her at all, Lvl 1 inflict wounds does up to 30 damage and gets advantage bonus to hit from backstab. Don't waste her spells on weak minions, just walk straight up to and behind the boss and kill them in 2 turns with inflict wounds backstab. With Shield of Faith and Lae'zels armor she has 21 AC, no need to worry about her being attacked.
