From the battle college:
Pulse Lighting - Zap someone for d3 POW 11 hits. Note that all three apply at once so that they a: all count to the same column or branch, b: the damage can only be shunted to one place by a warlock and c: only trigger effects that occur when the target is damaged once. Excellent assasination spell against lightly armoured warcasters. You can also boost some or all of the damage rolls separately. Fairly good range.
So apply the
ARM each time. The apply the damage all at once thing refers to
effects that trigger on damage, damage allocation on a warbeast/jack, etc.