As written you cannot use Selective Spell with Detonate. Selective Spell requires a duration of ‘Instantaneous’ and Detonate has a duration of ‘1 Round, then Instantaneous.’ While instantaneous is *included* in the duration of the spell, the duration of the spell is not ‘instantaneous.’
That aside, if your GM wants to overlook it, you would benefit from Selective Spell. The language in Detonate needs to be taken in context. The point of “You automatically take half damage from the explosion, without a Saving Throw” within its context is ruling that the caster does not get a saving throw for his own spell, but he only takes half the damage instead, because it makes no sense that you could “reflex” your way out of an explosion that is emanating from your own body. Then it goes on to say “but any other energy resistance or energy immunity effects you may have in place can prevent or lessen this overflow damage caused by the explosion.”
This is not restrictive language, it’s example-giving language. It’s not saying “only energy resistance or immunity effects can prevent or lesson,” it’s giving precedence for all sorts of magical immunity to the spell. Given that selective spell grants magical exclusion to a spell, It’s well within the intention of both “Detonate” and “Selective Spell.”
Lastly, no you can’t “get out of the AoE of the explosion.” The spell isn’t centered on your body, it’s coming from it. “You flood yourself with a potent surge of elemental energy.
One round after completing the casting of the spell, the energy explodes from your body.”
Even if you somehow managed to cast a teleportation spell or be the target of one as the explosion happened, the point of the spell is that it’s an overflow of magic energy *leaving your body*. So in a role play scenario, rather than rigidly action-based, you are taking the damage right before the enemy is. The energy bursts out of your body, damaging you, then floods the area of the radius of the spell, damaging anyone who doesn’t reflex out of the way
