Line for bigshot. I need help

[Godsanvil]

I am trying to channel 502 once then channel 903 until its dead.

This is what I have based on instructions in bigshot
prep 502 channel target, stance off and prep 903 channel target(xx)

This is what I get
[bigshot]>prep 502 channel #32126331
You intone a phrase of elemental power while raising your hands, invoking Chromatic Circle...
Your spell is ready.
>
[bigshot]>stance off
You are now in an offensive stance.
>
[bigshot]>prep 903 channel #32126331
You already have a spell readied! You must RELEASE it if you wish to prepare another!

I know "prep 502 channel target" works by its self.

Can anyone help me make this work?

[SashaFierce]

Try using incant set instead.

[Godsanvil]

This is kind of working. But it is just swapping back and forth between the spells and stays in offensive the whole fight.
prep 502, channel target(x1), prep 903, evoke target(xx)

[SashaFierce]

incant set 502 channel
incant set 903 evoke


502 target(x1), 903 target(x10)

[Godsanvil]

I give up I will just stick with 502
I can live with this
A zombie is suddenly surrounded by a circle of flickering flames.
CS: +133 - TD: +82 + CvA: +25 + d100: +83 == +159
Warding failed!
The flickering flames quickly swirl around the zombie, causing 23 points of damage!
... 70 points of damage!
Fire completely surrounds a zombie. Blood boils and heart stops.
The zombie falls to the ground, a lifeless lump of flesh.
Dead is dead.