The ability "At the beginning of the next end step, destroy that creature if it attacked this turn" expresses a triggered ability, more specifically a delayed triggered ability (C.R. 603.1, 603.7). Like most other triggered abilities, players get priority to activate abilities like River Boa's activated ability in response to it (that is, before it resolves) (C.R. 117.7, 117.1b).
if I play 2 berserks on the boa, can I regenerate it twice? do I need to?
In general, yes and yes.
If two different Berserk spells resolve for the same creature while it's attacking, two different abilities of the form "At the beginning of the next end step, destroy that creature if it attacked this turn" will trigger as the same "next end step" begins, and will each try to destroy that creature at separate moments if that creature "attacked this turn". However, in general, regenerating a creature replaces what happens only "the next time [that creature] would be destroyed this turn" (C.R. 701.15a), so that if something would destroy that creature, regeneration kicks in and is "used up", so that to keep the creature from being destroyed, there must be some other way to achieve that, such as another regeneration effect on the creature (C.R. 701.15a). See also this thread.
let's say I have a river boa in play.
I use berserk on the boa, while I'm attacking with it.
when it comes time to destroy the boa from berserk, I can pay a green mana to regenerate the boa, correct?
if I play 2 berserks on the boa, can I regenerate it twice? do I need to?,
The Precious
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In general, yes and yes.
If two different Berserk spells resolve for the same creature while it's attacking, two different abilities of the form "At the beginning of the next end step, destroy that creature if it attacked this turn" will trigger as the same "next end step" begins, and will each try to destroy that creature at separate moments if that creature "attacked this turn". However, in general, regenerating a creature replaces what happens only "the next time [that creature] would be destroyed this turn" (C.R. 701.15a), so that if something would destroy that creature, regeneration kicks in and is "used up", so that to keep the creature from being destroyed, there must be some other way to achieve that, such as another regeneration effect on the creature (C.R. 701.15a). See also this thread.
The Precious
B___U___G___R___W