I would probably make a macro action for each spell.

So instead of multiple Keystroke Actions on release.
It would be multiple Do Mapped Key actions, where each mapped key was an IS Boxer macro action.

Spam 1
Step 1 (Down)
- FTL Assist Me, Target: Current Window
- Send IWT keybind to Target: Group (all of) Melee ATG
- Do mapped key action - Howling Blast, Target: Group (all of) DK's

Spam 1
Step 2 (Up)
- Do Mapped Key Action - Howling Blast, Target: Group (all of) DK's
- Do Mapped Key Action - Frost Strike, Target: Group (all of) DK's
- Etc

Then your warcraft macros, within IS Boxer:

Howling Blast mapped key.
Keystroke Actions > Warcraft Macro Action
/Cast Howling Blast

You wouldn't need assist in the macros, because the mapped key is doing it already on Key Down, which you'll be spamming.

My macros don't reside within IS Boxer for the most part.
If anyone has a better approach, feel free to pipe in.