Hitting monsters with acid arrow, then swapping to melee weapon I figured out I am doing increased dmg. Here is exactly how its happening.
Was fighting with treant (tested 3 times), started to shoot it with LB with acid arrows triggered, when in melee range, swapped to halberd with bash skill triggered, increase to dmg was around 100% (+-300 -> +- 600). BUT, relenteless skill looks like its not working on it at all (never did same "max" dmg), and I wasnt able to reproduce first situation, when I hitted treant for around 1100 dmg. (I am not sure if it was thanks to 200% dmg melee skill heavy blow). No crits - my crit is around... -4%
On earth elementals it wasnt working tho, so I figured out it may be thanks to corrossion, as they are immune to it.