So I decided to sacrifice some Cerberuses in the first four battles of the Spire to get some testing data on this. (First four Spire encounters so that the enemies would be weak enough that they wouldn't completely obliterate my poor puppies before I got any usable data.) After a bit of this, I realized debuffs may have been interfering with my results, so I switched over to a low-level city and started twapping 2-star enemies with 1-star Vallorian Guards. (I deliberately walked them within Strike Back range.)
Conclusions:
- In my samples, I am seeing Strike Back damage anywhere from 32%-48% of the regular attack damage dealt by that unit. But that spread may simply be due to the regular random spread of damage, rather than an actual difference in the percentages themselves. Thus, you can think of Strike Back as doing roughly ~40% of the regular attack damage.
- The percent of damage that a Strike Back deals does not appear to be related to the type of unit attacking; I'm not (yet) seeing any signs that different units are better at Strike Back than others. (I've only tested Light Melee and Heavy Melee so far, however. I did test some enemy Treants, but so far not player Treants. Also, the standard random variance in damage may be throwing off my results in this regard.)
- Strike Back happens after the unit takes damage from the original attack, so the Strike Back damage is affected by unit loss from the original attack, and debuffs (if any) that the original attack applies.
- Strike Back never inflicts debuffs.
- Strike Back damage is affected by standard unit type bonuses/penalties (e.g. Light Melee against Heavy Melee or such). For example, if a unit has a -19% penalty when attacking a particular target, he'll have a -19% penalty on his Strike Back damage against that target also.
- Strike Back damage appears to be affected by debuffs, both an attack debuff on the unit using Strike Back, and defensive debuff on the target. In one example, the unit was getting a +33% damage bonus because of a defense debuff on his target; he got the exact same +33% on both his regular attacks and his Strike Back blows.
Trivia: The result of Strike Back damage is displayed on the unit portraits in the initiative bar at the bottom of the screen even before the animations play out. If a unit is going to get eliminated by Strike Back damage (suicide), you will see its HP go to 0 and its icon gray out at the bottom of the screen before the unit on the battle field visually even strikes its target.