Unit - Create 1 <Dummy lightning caster> at <Position of (Target unit of ability being cast)
Wait (Game-time) .3 seconds
Unit - Issue (Last created unit) to (Far Seer - Chain Lightning) <Random 1 unit within 300 range matching (Target unit of ability being cast not equal to Matching unit)>
Wait (Game-Time) .3 seconds
Unit - Remove (Last created unit) from the game
Something like that, but it would only bounce once... also if another unit spawns it will remove the wrong one so you may need a variable, also the waits are there because some spells, even without a casting time, still take a fraction of a second to cast which therefor wont cast and will be removed when its created.