Yu-Gi-Oh Millennium Duels

This achievement can be done in combination with the "World Duelist" achievement for playing 30 online matches and may assist in you making your way toward the "One Better" achievement for inflicting at least 1,000,001 damage if you end up going for this achievement by actually playing. I will however caution that online games played and wins only count if the game is played to its completion meaning that if someone quits out midway then the game doesn't count.

Please note that there are two sections to this solution, one for boosting and for not boosting. I hope, whichever one applies to you, that it helps in your unlocking of the achievement!

Also, no matter which route you take, you can keep track of your statistics by going, from the main menu, to "Help & Options" then to "Statistics" which will have a list with total damage, total multiplayer matches and total ranked wins.
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If boosting

The quickest method, which requires two players including yourself and will unlock the play 30 matches achievement, is as follows:

Go to "multiplayer" then select "Ranked Match" and choose to "Create Match" with the following settings (the change you have to make to the settings are in bold):

Select Duel Mode: Normal
Match Settings: Single
LP: 2000
Time Limit: 30
Deck Select: (Will be set to whatever your main deck is listed as)

Then select "Create" and have your boosting partner navigate to the game session (they are listed by gamertag so it should be really easy to find). If someone else joins your game (there are generally 5 unique gamertags online at any given time) then the host will have to remake the game session.

Once the host starts the game there is a single rock-paper-scissors sequence that allows the winner to choose whether they go first or second and you'll want to coordinate with your boosting partner (for speed and efficiency) to allow the person who is winning this specific match to go second.

The game begins with the losing player drawing and then he/she waits 30 seconds at which point the game will award the other person the win due to you running out of time.

It is worth noting that you, unlike in the single player, are not able to surrender on your first turn and thus we use this method which takes about 1 minute per game allowing you to do 30 matches in about half an hour depending on the the amount of people online.
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If not boosting

You can still use the quickest match settings (as described in the section above), but you must use patience and make sure you've built a competitive deck before venturing online.

In addition to grinding out cards in the single player, importing cards from your 5D's save file (and the available DLC for that title) has a significant impact on reducing the time required to become competitive.

If you are not a Yu-Gi-Oh player, or just want a little help, then I highly recommend taking advantage of the countless online resources to build your deck. A simple google search will yield dozens of lists and videos of builds for highly competitive decks (be warned, the ban list and availability of cards are assuredly going to be slightly different, especially as this game gets older and older) and often come coupled with explanations on how the decks work.

Some great resources for people looking to become better deck builders (or just create better decks) are: https://yugiohblog.konami.com/?tag=top-decks which provides the names of decks that have recently done well at regional/national Yu-Gi-Oh competitions as well as http://yugioh.wikia.com/wiki/Staple and http://yugioh.wikia.com/wiki/Semi-staple which provide lists of staple and semi-staple cards that you undoubtedly will find in the majority of decks online.

The time required for this method varies as it depends on the competitiveness of your deck and how it generally attains victory (One Turn Kill Decks often hit hard and fast allowing for quick victories/defeats whereas other competitive decks tend to slow the game down).

Although I would suspect it to take upwards of a 8-10 hours to complete this legitimately. And do be warned that if the previous iteration was any indication of online sportsmanship, then this game is going have rage quitters galore.
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Best of luck, and if there is anything that isn't clear in the solution or that should be added then please let me know and I'll be sure to modify it as best I can!

This achievement can be done in combination with the "World Duelist" achievement for playing 30 online matches and may assist in you making your way toward the "One Better" achievement for inflicting at least 1,000,001 damage if you end up going for this achievement by actually playing. I will however caution that online games played and wins only count if the game is played to its completion meaning that if someone quits out midway then the game doesn't count.

Please note that there are two sections to this solution, one for boosting and for not boosting. I hope, whichever one applies to you, that it helps in your unlocking of the achievement!

Also, no matter which route you take, you can keep track of your statistics by going, from the main menu, to "Help & Options" then to "Statistics" which will have a list with total damage, total multiplayer matches and total ranked wins.
-------

If boosting

The quickest method, which requires two players including yourself and will unlock the play 30 matches achievement, is as follows:

Go to "multiplayer" then select "Ranked Match" and choose to "Create Match" with the following settings (the change you have to make to the settings are in bold):

Select Duel Mode: Normal
Match Settings: Single
LP: 2000
Time Limit: 30
Deck Select: (Will be set to whatever your main deck is listed as)

Then select "Create" and have your boosting partner navigate to the game session (they are listed by gamertag so it should be really easy to find). If someone else joins your game (there are generally 5 unique gamertags online at any given time) then the host will have to remake the game session.

Once the host starts the game there is a single rock-paper-scissors sequence that allows the winner to choose whether they go first or second and you'll want to coordinate with your boosting partner (for speed and efficiency) to allow the person who is winning this specific match to go second.

The game begins with the losing player drawing and then he/she waits 30 seconds at which point the game will award the other person the win due to you running out of time.

It is worth noting that you, unlike in the single player, are not able to surrender on your first turn and thus we use this method which takes about 1 minute per game allowing you to do 30 matches in about half an hour depending on the the amount of people online.
-------

If not boosting

You can still use the quickest match settings (as described in the section above), but you must use patience and make sure you've built a competitive deck before venturing online.

In addition to grinding out cards in the single player, importing cards from your 5D's save file (and the available DLC for that title) has a significant impact on reducing the time required to become competitive.

If you are not a Yu-Gi-Oh player, or just want a little help, then I highly recommend taking advantage of the countless online resources to build your deck. A simple google search will yield dozens of lists and videos of builds for highly competitive decks (be warned, the ban list and availability of cards are assuredly going to be slightly different, especially as this game gets older and older) and often come coupled with explanations on how the decks work.

Some great resources for people looking to become better deck builders (or just create better decks) are: https://yugiohblog.konami.com/?tag=top-decks which provides the names of decks that have recently done well at regional/national Yu-Gi-Oh competitions as well as http://yugioh.wikia.com/wiki/Staple and http://yugioh.wikia.com/wiki/Semi-staple which provide lists of staple and semi-staple cards that you undoubtedly will find in the majority of decks online.

The time required for this method varies as it depends on the competitiveness of your deck and how it generally attains victory (One Turn Kill Decks often hit hard and fast allowing for quick victories/defeats whereas other competitive decks tend to slow the game down).

Although I would suspect it to take upwards of a 8-10 hours to complete this legitimately. And do be warned that if the previous iteration was any indication of online sportsmanship, then this game is going have rage quitters galore.
---

Best of luck, and if there is anything that isn't clear in the solution or that should be added then please let me know and I'll be sure to modify it as best I can!

Here is an even faster way. Have the host create a duel with a time limit of 30 seconds. Make sure the person whose turn it is to lose goes first. Just wait out the clock and the win will count.