[Mathias]

 
  <thing id="pS2AspStag" name="Aspect of the Stag" description="When you cast this spell, you take on an aspect of a stag, including some of its physical characteristics. Your features become elongated and sinewy, and you grow a set of antlers you can use for defense. You gain a +2 dodge bonus to AC against attacks of opportunity, your base speed increases by 20 ft., you can move through any undergrowth (including magically manipulated undergrowth) at your normal speed, and can even make a 5-foot step within such terrain. Furthermore, when you are hit with an attack of opportunity, you can make a single attack with your antlers against the opponent that hit you as an immediate action. This attack uses your highest base attack bonus plus your Strength or Dexterity bonus (your choice) and deals 1d8 points of piercing damage (if you are Medium; 1d6 points of damage if Small) plus your Strength modifier on a successful hit. The antlers have a critical multiplier of 19-20/×2." compset="InPlay">
    <fieldval field="pDuration" value="1 min/level"/>
    <tag group="Helper" tag="AdjSpell" name="Spell Adjustment" abbrev="Spell Adjustment"/>
    <tag group="Helper" tag="NoIncr"/>
    <bootstrap thing="wGore">
      <containerreq phase="First" priority="500"><![CDATA[
        fieldval:pIsOn <> 0]]></containerreq>
      <autotag group="wMain" tag="1d8_6"/>
      <autotag group="Helper" tag="NatSizeDmg"/>
      <autotag group="Broadcast" tag="ImpCrit"/>
      <assignval field="livename" value="Antlers"/>
      </bootstrap>
    <eval phase="Final" priority="9900"><![CDATA[~Final Phase 9900
~if we're not active, just get out now
doneif (field[pIsOn].value = 0)
hero.child[Speed].field[tSpeedMod].value += 20]]></eval>
    </thing>

The Helper.NatSizeDmg mechanic on a natural attack means to add or subtract as many damage steps from the damage of the weapon as the hero is from size M - presumably, a size L character getting this spell would get a 2d6 gore attack, and a Tiny character would get a 1d4 - not just the small/medium cases described in the spell.

Tags can be attached directly to a bootstrap - that way, you can avoid altering the spell with an Eval script. The tags button in the editor can be used to add any tag that exists.

The contanierreq I've added is how to make a bootstrap only be live while the adjustment is active. In the editor, while editing the bootstrap, enter:

 
fieldval:pIsOn <> 0

In the Condition button, at phase: First, Priority: 500.

Since this is an APG spell, I've also assigned it to the APG source.

I had a question - the minimum and maximum - your code had spaces for the values there - did you enter spaces in the editor to remove the text that was there? If so, delete the text instead - those fields are numerical fields, so it's best not to have text (like " "), even if HL can deal with it properly.