This is a system transition issue, not a creativity issue.
4e is a very different system and that's okay, but it's not for everyone. There's a gap between the player and the system and your job as GM is to help facilitate bridging that gap. Your goal in this should not be to make the player conform to the system, but to help the player understand the system's approach enough to decide if the system is one she wants to play in.
It's important to realize that creativity is not 'at stake' here: you can be just as creative in 4e as in any other system... just not in exactly the same fields.
What's the difference?
4e instituted multiple ground-level philosophy changes that build on each other:
Parties are hardy and self-contained. Instead of needing to exploit their environments and using dirty tricks to desperately survive on 3 hp and exceptionally limited healing, even the lowest-level 4e party has everything it needs to survive and triumph all written there on the character sheets.
It's not the net that's special: it's the person throwing it. 4e has feats, features, powers, and magic enchantments which could let a PC mimic that power's restraining effect. The net is probably the least important part of the equation; in 4e items are important and necessary but rarely definitive.
Realism takes a back seat to modular balance. Early editions left a great deal of mechanics to what the GM thought felt right, and 3.5 introduced numerous subsystems to provide rules for the same effect. This led to (often unexpected) problems with silly combinations and massive power imbalance. In order to reduce these problems, 4e abandoned most of its predecessors' attempts to mimic a 'real world' feel to its mechanics: it replaced them with standardized power gain, standardized effects, and a universal class system.
Together, means that rules are really important to 4e.
In earlier D&D editions, the player's line of reasoning would be absolutely correct: while a player has ability, most often it is the item the player wields which defines the effects the item can achieve. This was a major theme of D&D for a long time: the item makes the hero, and the heroes MUST grab every advantage they can or they will perish.
But in 4e, combat mechanics are balanced so carefully and already weighted so firmly in the players' favor that remaining within the game's mechanics is important to keep the game from spinning out of control. It's not your responsibility as a GM to make that happen invisibly; it's your responsibility to invite the players to work with you to make the game a good one.
To this end, player creativity in 4e needs to be channeled (voluntarily by the players, with support from the GM) away from scrounging and toward creative application of the existing rules. Finding how effective the nets are might make the PC want to start taking feats and features to learn how to use them, for example.
4e's fun is inside the box
4e is appealing to two major groups of players:
- Those who enjoy taking a structured system and making it perform to the extremes of its potential.
- Those who enjoy working in a system where even the least CharOpped builds are capable of achieving a modicum of success.
Other players may or may not find its modular philosophy chafing, but remember that any system can support strong, creative role-playing... just not always in the same areas. It's important for everyone at the table to be on the same page about the kind of system you're playing, or misunderstandings like your nets will crop up.
Here're the basics:
- Have the player sandbag. Explain to the player that his optimized PC makes DMing too difficult. The problem isn't that the player's winning—the DM, after all, has infinite monsters—but that the player's character is overshadowing the other players' characters. Strongly urge the player to pick a character class 1 to 2 tiers lower than the class of the highest or lowest tier PC.
- Have the player optimize other PCs. The player may be expecting other players to design characters of similar stature. However, other players may have different strengths and priorities beyond character optimization. But if he and the other players agree, it's reasonable to have the optimizer make other players' characters for them, putting them all on the same metaphorical level. The DM'll have to adjust challenges appropriately, however.
- Avoid conflict and suffer. Steel yourself to spending an inordinate amount of time customizing encounters for a single self-absorbed murder-machine and his party of lickspittles. During play, fudge die rolls to keep alive the lickspittles while still challenging the murder-machine. I advise against this, but it is an option. Good luck.
Now that those're out of the way.
Encourage the player to optimize his character for tasks other than monster-killing
Although this isn't an obvious path for a character to take in Dungeons and Dragons 3.5, such a character can be a interesting experiment when played by an experienced, squeeze-every-ounce-from-the-system player.
It's extremely easy to make a 3.5 character who can kill anything. It's much harder, for example, to make a character who can steal anything or heal anything. Explain to the player that everybody knows he can already play a character who can murder the campaign world and that playing that again wastes his talents. Have him turn his eye to little used subsystems, oft-ignored fringe mechanics, and seemingly-hard-to-optimize game elements.
Bear in mind, though, that he will use whatever he specializes in to try to solve every problem. Currently, this isn't that big of a deal—killing monsters solves a lot of problems—, but, if the specialty is extremely obscure, group cohesion and, subsequently, the campaign may suffer.
Mandate group cohesion
If the character is forced through mechanics or story to care about his fellow PCs, he should stop charging into rooms alone and murdering information-spouting NPCs.
Have him designate another PC as his brother, sister, childhood friend, lover, confessor, ward, or something, and tell him his character cares about that person deeply. If this a party of random murderhobos who met in a bar and decided to go camping forever, maybe the gods, fate, or prophecy binds them, and breaking that bond means Certain Doom.TM
The player needs a reason for his character to keep associating with these characters who are so obviously beneath him, and the player won't give his character this reason unless there's a mechanic that makes his character better because his character has that reason.1
Developing that reason, then, becomes the DM's job. The player probably won't like this mandate as it makes his character vulnerable in a way that mechanics can't usually minimize, but once the player starts engendering some goodwill from the other players by optimizing his character's ability to keep his friends alive and get along with the group in addition to his character's ability to kill faster (although, admittedly, that, too, can keep his friends alive), the player may have a more enjoyable experience playing that character than playing his typical brooding, uncaring lone wolf who only looks out for himself.
Note
I GMed for the win-at-any-cost player for a decade. It's trying but does improve one's GMing skills. I've GMed the player—the same player—who thought the game would be better as a solo campaign and saw him come around to the idea of group cohesion after mandating it before the campaign began. Since I put forth that mandate in that campaign, I've done the same, to lesser and greater extents, in every campaign after. There's power to be had in saying A character who can't get along with the party isn't in the party and ending the conversation. Does saying…
- that the three months spent in a life raft created between the characters unbreakable bonds that will last the rest of the characters' lives, or
- that the characters' shared affection for their homeland despite their differing alignments unites them against common enemies, or
- that the royal family plucked the characters from obscurity to employ them as troubleshooters therefore the characters owe the royal family their unwavering service
…remove some agency? Sure. But, afterward, you can play the game. Role-playing games are (usually) cooperative affairs, and a player thinking his deliberately obstructionist, shenanigans-pulling character is acceptable makes the game a drag.
1 For this same reason, I strongly suspect all his characters but those needing to be otherwise are that least vulnerable of all the alignments, True Neutral.
Best Answer
Correct.
Incorrect. Unless the Earth Elemental takes the hide action, its location is known. The fact that it can move through the earth without disturbing it doesn't mean that it can do so without giving its position away - there may be visual, audible or other sensory clues as to where it is but however you decide to narrate it, something reveals its location.
Unseen is not the same as hidden. See What advantages does hiding have?
Incorrect. You can only be surprised at the start of the combat. See How often during combat can you be Surprised?
Incorrect.
First, being surprised does not give the attacker advantage.
Second, while unseen attackers get advantage, however, the Earth Elemental must leave the earth in order to attack and is therefore not unseen when it attacks.
Correct. Apart from the advantage as discussed above.
See above.
Game breaking? Hardly.
A potentially problematic encounter that requires the PCs to develop novel tactics on the fly? Sure, but that's a good thing.
One strategy that my players used was to simply Ready (ranged) attacks and spells triggered "when an Earth Elemental leaves the Earth." 4-5 PCs wailing on a single Earth Elemental each time it sticks its protuberances out soon turns it into gravel.