Strictly better Withstand Death.

That's not saying much.

It's a simple note - similar to those mentioning that the card is a reprint - so that those feeling compelled to point that out don't need to do so. I'm not 100 % of creating a strictly better version of Withstand Death since it's a "solid" card though it doesn't see extensive play in any competitive format.

Other cards comparable to this could be Might of Oaks and Enlarge. Originally the card gave +7/+0 when proclaimed, but I chose against that since flexibility is very powerful in itself.

Wording advice I took from Vampire's Bite.

Perhaps the most interesting quality here is that this is one of the few cards that only pump power. However, that makes sense when combined with indestructible since buffing toughness would be somewhat redundant. Still cards such as Bitter Cold could be used to take it down - even after this spell has resolved.

Like most proclaim cards, I've opted to make this an instant, since that forces a decision of some sort (since proclaiming is done at sorcery speed). Some of the proclaim cards, such as Coming of the Firstborn, asks you to abandon the surprise element in exchange for a greater effect. In this card that plays a bit differently since it pretty much becomes an entirely different card when proclaimed. It still has the decision of whether you want to pay in one go (on your turn), or pay first hand, which might make the opponent keep their mana up for your next turn so that they can shoot the target down in response when you finally actually cast the spell. Though even if you pay you could keep the spell stored in your hand for a while and even use it defensively instead which, well... There are small decisions sprinkled all over with a mechanic like this.

Also I could go on about where the name (flavor) stems from but I think I come back to that later on.

The reveal on proclaim is part of the cost for the effect, yes? So if you reveal the card and say you're proclaiming it, I shoot down your dude in response, you could you cast it for ? Trying to understand exactly how the timing of this ability works.

Yeah, it's activated from a hidden zone so it has to be revealed as part of the "cost" (as far as I understand it).

+6/+0 doesn't feel very green. The creature has indestructible so you may as well give it +6/+6.

Good card btw.

The way I see it, it's pointless to give a toughness boost when you give indestructible as well. I like the added dimension that this won't help against the various -X/-X spells.

Increasing the toughness cannot at the same time be "pointless" and an "added dimension" in being weak against -X/-X effects.

Okay, let me rephrase that: It's pointless in that it unnecessary removes an in-built weakness indestructible effects have. + I think the card is cooler this way.