Dorkster is the authority here, but i already asked something similar. The answer is the game cant track if the shield is on or not; that means that you need to make Shield + postpower of X time  to achieve something similar to what you want.

You would give "shield" to player, apply counter-damage X time, but i dont think the game makes distinction on damage received to the shield (it does to the player, hence elemental resistances), so the shield only has one kind of hp that takes full damage from everything(? need testing), you could raise elemental resistances X time when casting shield, i already did it like that, have a look. As for dealing damage to attackers, its not possible in direct ways; the postpower cant aim damage spells to enemies, so you would need to use either a spell with random target around player (look caltrops, or any magic that fires in defined patterns like my final version of freeze) or an area of effect spell centered on player (quake or burn).

You need to declare a postpower into it:

post_power=2629

Then something like this (power from mod different to mine):

[power]
id=2629

name=Cadaverous Explosion
type=fixed
icon=37
description=Make a corpse explode, damaging nearby enemies.
new_state=cast
face=true
requires_los=false
requires_targeting=true
animation=animations/powers/blast.txt
soundfx=soundfx/powers/burn.ogg
use_hazard=true
base_damage=ment
lifespan=600ms
radius=2.0
floor=true
trait_elemental=dark
starting_pos=source
multitarget=true
post_power=158
target_range=8.0
requires_corpse=consume

Erase requiere corpse/trait_elemental to fire and add:

count=40 (calculate how many times it will fire and change value)

delay=1500ms

count=40 ->Total number of times the power will fire

delay=1500ms -> delay between triggers

So every 1.5 seconds a "burn" fires, up to a total of 40 times. So desired time / interval  = result total number of times it needs to shoot to acomodate to power.

Try to delete:

new_state=cast

And see if it works to avoid cast animation, but i think it will only appear the first time.

While you can do powers that poison (not much different to bleed) i have no idea about one that lingers around. Maybe its not that difficult, and it just need count and delay like before and some new graphic; why dont you try and tell me?

The very same power you are creating but intead of starting_pos=source change to starting_pos=target

It should create a burn centered at mouse, that fires 40 times each 1.5 seconds; if it goes well add post-effect poison, if it works, its just a matter of getting a new particle effect graphic of a green cloud.