If I target an enemy unit at a BF Wot Defiant Dance and the opponent reacts killing its unit, do I still get +2 from Defiant Dance?

If you target an enemy unit with Defiant Dance and that unit is killed by a reaction before the spell resolves, you will still receive the +2 Might effect on your other unit.

Explanation

According to Rule 356.3.e.5, if any of a spell's targets are no longer legal when the spell resolves, those specific targets are unaffected by the spell. Furthermore, Rule 356.3.e.8 states that if an instruction has multiple targets and some become invalid, the instruction will execute only on the remaining valid targets.

In the case of Defiant Dance, the card text requires you to choose two distinct units: one to receive +2 Might and another to receive -2 Might. Because the spell's instructions are linked to these specific targets, if one target becomes illegal (e.g., it is removed from the board), the instruction to apply the effect to that target cannot be executed.

The spell will still resolve, applying its effects to the remaining valid target. Since the +2 Might and -2 Might are distinct parts of the spell's instructions, the loss of one target means that specific portion of the effect fails to execute, while the remaining valid target receives its intended effect.

Note: I could not fully verify this ruling against the retrieved rules.