LayersThe Comprehensive Rules wrote:
613.1a Layer 1: Copy effects are applied. See rule 706, “Copying Objects.”
613.1b Layer 2: Control-changing effects are applied.
613.1c Layer 3: Text-changing effects are applied. See rule 612, “Text-Changing Effects.”
613.1d Layer 4: Type-changing effects are applied. These include effects that change an object’s card type, subtype, and/or supertype.
613.1e Layer 5: Color-changing effects are applied.
613.1f Layer 6: Ability-adding effects, ability-removing effects, and effects that say an object can’t have an ability are applied.
613.1g Layer 7: Power- and/or toughness-changing effects are applied.
The only layer that applies here is Layer 7, which further breaks down to:
The Comprehensive Rules also wrote:
613.3a Layer 7a: Effects from characteristic-defining abilities that define power and/or toughness are applied. See rule 604.3.
613.3b Layer 7b: Effects that set power and/or toughness to a specific number or value are applied. Effects that refer to the base power and/or toughness of a creature apply in this layer.
613.3c Layer 7c: Effects that modify power and/or toughness (but don’t set power and/or toughness to a specific number or value) are applied.
613.3d Layer 7d: Power and/or toughness changes from counters are applied. See rule 121, “Counters.”
613.3e Layer 7e: Effects that switch a creature’s power and toughness are applied. Such effects take the value of power and apply it to the creature’s toughness, and take the value of toughness and apply it to the creature’s power.
Here we're looking at layer 7c, and 7e. The important thing is that things in layers are
always applied in the order listed above.
So you have a 1/3 Fire Urchin.
You cast Invert. Fire Urchin's ability triggers, and gives it +1/+0 (2/3). This is layer 7c
Invert resolves. Power and toughness are switched. (3/2) This is layer 7e.
Cast Sure Strike. Fire Urchin's ability triggers again, and gives it +1/+0. Now this is where it gets a little weird; what the game sees here is a 1/3, with +2/+0, switched,
in that order so it's a 3/3, switched (still a 3/3)
Sure Strike resolves, giving Urchin +3/+0. This
also happens in layer 7c. So you have a 1/3, with +5/+0, switched. You have a 3/6.
The answer is, Fire Urchin is a 3/6. Invert is bad for Fire Urchin.