vic: Sorry, I thought you were still asking about normal fight effects, not Arena's specific behavior. But now it seems like we need to clarify what happens when you make a copy of a spell or ability.

Making a copy of a spell or ability on the stack creates an exact copy that accounts for any choices made during the casting process, including modes, targets, distribution of counters/damage and other such things. This is why every spell/ability copying effect will allow you to choose new targets for the copies (Doublecast, Illusionist's Bracers, etc.) or will tell you something specific about what to do with the targets for the copies (Radiate, Ink-Treader Nephilim, etc.), otherwise copy effects wouldn't be half as useful or interesting. There are no Ink-Treader style effects in the game that can copy a spell/ability with more than one target, meaning you would be using an effect that allows you to choose new targets for the copies. Your opponent wouldn't get a say in it.

To the bigger question you might be asking about what happens when different players need to make choices about something simultaneously: first the active player makes their choices and then the non-active player makes their choices. If there are more than 2 players in the game, then it starts with the active player and goes around the table in turn order. But if your Ink-Treader Nephilim makes a bunch of copies of something, you're the only one who will be making decisions for the order that the copies go onto the stack because you control them all.

Like I said earlier, if you make a bunch of copies of a spell simultaneously with an effect you control, then you control the copies and will decide what order they get put onto the stack. They will resolve one at a time according to the order you choose.

If that spell is a "Fight" effect, then one pair of creatures will fight, then another then another and the order is based on what you decided earlier. If you get to a point where one of the creatures in a pair has already died to one of the fights that already resolved, then that "Fight" effect does nothing and you move on.

This specific scenario can't happen. Prey Upon has two targets, so Ink-Treader Nephilim's ability won't trigger for it.

If you were to somehow make a bunch of copies of a spell simultaneously with an effect you control, you would also control the copies and would decide what order they get put onto the stack. They would resolve one at a time according to the order you chose

In the case of multiple copies of Prey Upon resolving, each fight would be handled individually as each Prey Upon resolves. If a creature is the target of multiple Prey Upons and happens to die after one of the earlier ones, any later ones that targeted it will do nothing when they start resolving because a "fight" effect does nothing if one or both of the creatures are no longer on the battlefield or have otherwise become illegal targets.

Lumbae: A response to your question has been up for a while. Since there don't seem to be any follow-ups or corrections that need to be made, I marked it as the "Accepted answer" so this topic can move out of the list of unanswered questions. In the future you can take care of this yourself using the "Mark as Answer" button on the response that you feel is the most helpful answer to your question.

I try to avoid marking my own responses when I do this but in this case it was the only option.

@bhst5019: Responses to your question have been up for a while. There don't seem to be any more follow-ups or corrections that need to be made, but since there was some confusion on this initially I'm going to leave it open for a couple more days to give you a chance to mark what you feel is the most helpful response as the "Accepted answer". Feel free to respond with any more follow-up questions if you don't feel like it's been resolved.

It won't count. If an effect calls out the name of a permanent type (creature, enchantment etc.) but doesn't also specify something like "spell", "card in a graveyard" etc. then that means it's only talking about a permanent of that type on the battlefield. Vren's ability will only consider things that get exiled from the battlefield.

Some additional backup info: In older versions of the rules (Alpha through 5th Edition), if a batch or series started resolving, players wouldn't get priority again until it was completely finished. However, in the 6th Edition rules update that introduced the stack, spells and abilities on the stack resolve one at a time and priority goes around the table again after each one resolves. The next object on the stack can only start resolving if all players pass priority in succession.

There's a big problem with what your opponent is trying to do. Frenetic Sliver's ability specifically says you only flip the coin and start doing the rest "if this permanent is on the battlefield" (current Oracle text). After a single one of the activations resolves, the creature is either dead or in exile, so the rest of the activations still on the stack will do nothing.

Let's say your opponent is using Frenetic Efreet instead, whose current Oracle text has the exploit that the Sliver was specifically written to disallow. In that case, what your opponent is trying to do still doesn't work. The result of the coin flip is applied as part of the resolution of the ability, in which case the Efreet will either be dead or phased out and can't be activated any more this turn. If your opponent wants to add multiple activations to the stack in this example, they have to do it all at once and they have to specify exactly how many. Also, game shortcuts can only be used to skip through repeated events with definite outcomes, not chance-based ones, so your opponent will need to handle the resolution of each activation individually and abide by the results.

You're right, I didn't consider the copy of the first Ulalek trigger that would be made if you left it on the stack. Using that, you could keep going with making copies as long as you had another to spend each time. As long as the initial Ulalek trigger is still waiting on the stack, any Ulalek trigger that you pay for will make a copy of it, including those copies themselves.

"SBA" stands for "State Based Action". "State Based Effect" is not an official rules term.

The SBA to send the commander to the Command zone after being put into Exile is optional. You can leave the commander in Exile if you want.

When you say "token", it's not clear whether you mean the token copies of Glaring Fleshraker and Propagator Drone that are created by Ulalek, Fused Atrocity's trigger or you mean the Eldrazi Spawn tokens created by Fleshraker's trigger.

This is what you do to maximize the total copies of both Fleshraker and Drone, assuming you want more Fleshrakers:
1. Cast Fleshraker. Ulalek's ability triggers
2. Pay for Ulalek's trigger. You get a copy of the Fleshraker spell
3. Before the copy of the Fleshraker spell starts resolving, respond by casting Drone. Ulalek's ability triggers
4. Pay for Ulalek's trigger. You get a copy of the original Fleshraker spell, the copy of the Fleshraker spell and the Drone spell
5. After everything resolves, you end up with 2 total Drones and 3 total Fleshrakers

Fleshraker does not trigger in this example because it was not on the battlefield while any spells were being cast. If you want some Spawn from the Drone casting, you'll need to let one or both of the Fleshrakers resolve first before casting Drone.

This is what you do to maximize the number of Eldrazi Spawn tokens made from Fleshraker's trigger:
1. Cast Fleshraker. Ulalek's ability triggers
2. Pay for Ulalek's trigger. You get a copy of the Fleshraker spell. Let them both resolve
3. Cast Drone. Ulalek triggers and both Fleshrakers trigger. Order the triggers with Ulalek's on top so it will start resolving first
4. Pay for Ulalek's trigger. You get a copy of the Drone spell and a copy of both Fleshraker triggers
5. After everything resolves, you get the original Drone, a copy of the Drone and a total of 4 Eldrazi Spawn tokens

Note that creating a copy of the Drone spell does not trigger Fleshraker again because the copy isn't being "cast", it's just being created directly on the stack.