The art has small attack power, so it is very luck based if it do damage, but boosters really help, over-strength adds more attack power (not much, but helps), as xp, and the difficulty gives them a free star worse of xp on bonus. Since art protected (usually) it can get quite some xp, 2-3 stars, with the bonus from difficulty, and a few point of over-strength gives it higher chance for damage. Also, while morale and such is irrelevant for the morale and de-entrenchment, for damage, it is calculated, so a well supplied and cared gun will do more damage than a barely helped one. The other big factor is the defending units tech lvl, if your are not up to date, it will suffer much more damage.
Elite also preserves the xp of the art, since you only lose if you normally reinforce, so an 11-12 art taking 1-2 damage is preserved, while a 10 taking the same will lose lots of xp at reinforcing.