I was told to come here to get some advice. I can't beat Harpies no matter what I do, most of the cards I'm recommended to side I'm either already siding, or they're already in my main deck to begin with, and any cards I'm NOT already siding or are in my main deck, I can't find a place for anyway since my side deck is maxed out with cards I need to beat Blackwings, Bujins, Burning Abyss, Constellars, and Frognarchs. Now normally, the solution here would be "build a different deck", but I've tried that. I got back into this game about four years ago, and in that time the ONLY and I mean ONLY deck I've ever been able to win consistently with has been Malefics. And to this day I'm still constantly trying, building decks, rebuilding decks, and they just never work. Only Malefics. So using my Malefics just to counter the decks listed while making another deck to counter everything else is just not an option apparantly. So somebody, please tell me what I'm doing wrong, if I AM doing anything wrong with:

Main Deck:
Malefic Truth Dragon x2
Malefic Cyber End Dragon x3
Beast King Barbaros x3
Malefic Stardust Dragon x3
Thunder King Rai-Oh
Marshmallon
Cardcar D
A Cat of Ill Omen
Raigeki
Dark Hole
Terraforming x2
Mystical Space Typhoon x2
Kaiser Colosseum x2
Burden of the Mighty
Dimensional Fissure
Mound of the Bound Creator x2
Malefic World x2
Necrovalley x2
The Seal of Orichalcos
Bottomless Trap Hole
Threatening Roar x2
Torrential Tribute
Compulsory Evacuation Device
Scrap-Iron Scarecrow x2
Skill Drain
Dark Illusion x2
Dark Bribe x2
Solemn Warning

Extra Deck:
Cyber End Dragon x3
Stardust Dragon x3
Number 95: Galaxy-Eyes Darkmatter Dragon
Number 107: Galaxy-Eyes Tachyon Dragon
Hieratic Sun Dragon Overlord of Heliopolis
Number 40: Gimmick Puppet of Strings
Divine Dragon Knight Felgrand
Number 15: Gimmick Puppet Giant Grinder

Side Deck:
Effect Veiler x2
Mystical Space Typhoon
Zombie World x2
Macro Cosmos
Light-Imprisoning Mirror x2
Mistake x2
Rivalry of Warlords x2
Shadow-Imprisoning Mirror x2
Dark Bribe

You decks appears to have a few issues. First you're running 45 cards, which is going to hurt you chances of drawing into anything specific, secondly you're running 1 copy of several cards which isn't going to make drawing them any easier, finally a few of your card choices are not really that good.

Some cards that I have comments in regards to

Marshmallon - It's not going to buy you more than a turn, if that, additionally this doesn't seem like a deck that you want to stall with.

Malefic Truth Dragon - The LP cost is probably irrelevant, but it's a situational card, is likely to be dropped on the opponent's turn (and thus not likely to survive), and has no real built in protection.

Cardcar D - It could be usable if you run 2-3, but I think you can make room for better choices, additionally it doesn't help you if you only have 1, since you are not likely to see it.

A Cat of Ill Omen - Not really fast enough, besides there is no reason to run this over a third terraforming.

Kaiser Colosseum - This seems more like a side deck cards, you could keep these as is if you want, it's still a strong card.

Burden of the Mighty - When you can drop 4000 ATK monsters almost whenever you want, this doesn't seem like something that will contribute much to winning duels.

Dimensional Fissure - Something I would side deck personally.

Mound of the Bound Creator x2 - Honestly this card I would probably run 3 of, it seems to be your best field spell.

Necrovalley - Another card I would probably commit to the side deck

Threatening Roar - Normally it's better to run cards that get rid of threats rather than just stalling them out

Dark Illusion - Seems kind of Situational, it might be usable but personally I'd go for a negation or removal card that doesn't have any limitations, even if it doesn't cover both monsters and spell/traps it is probably more likely to see use.

Some cards that I feel might be of use

Denko Sekka - Seems like you could use this to lock out your opponent's backrow, then drop your field and Malefic, if you also have the Seal of Orichalcos you could use the larger Malefic to cover this, there is also the options of Trap Stuns or a sided set of Royal Decrees which might be more reliable.

Forbidden Chalice - This gets your Beast Kings up to 3000, and if used on a Malefic you get a slight ATK boost and your other monsters can attack. On the other hand, the 400 ATK isn't likely to let your opponent's get over a Malefic, this is also damage step usable, and works well if you go with a sided Royal Decree rout.

Chicken Game - You should be running 3 Terraforming, so this can provide additional draw power if you draw Terraforming with an actual field spell. Just don't use this to keep your Malefic in play since the opponent can pay 1000 to get rid of this.

Upstart Goblin - Good for drawing into cards that you need.

Ghost Ogre & Snow Rabbit - This makes for a good side against certain decks, though with Malefic Stardust/Beast King you can also synchro for Star Eater.

Forbidden Scripture - This might be usable against certain decks, Bujin's especially. It would be quite difficult for anything to match a Malefic using only original ATK.

Summon Breaker - When I actually tested a deck out to see what suggestions I could make, I was able to wall Harpies behind this card with Malefic Stardust on board, mind you this is probably more against the AI as it had no problem triggering the effect each turn, however it still makes it very difficult to make Xyz plays against the dec.

And the Band Played On - Good for stopping Xyz Decks in their tracks.

Mind Drain - Good side for a few decks (Bujins and Nekroz off the top of my head) (Full disclosure: Side decking is the aspect of the game that I am the least experienced in)

If you're ok with OCG cards, Solemn Notice could probably see play in keeping your opponent under control, Twin Twister might also be good since it seem like you would have redundant Field Spells to use as discards.

Final note
I've attached the deck I was testing with to this post, I changed the main and side decks, though not the extra, I would probably drop number 40, and put in a Star Eater or two, maybe a Fully Armor Photon Dragon, and a second Felgrand. The deck itself probably needs more work as I just played a few games with it, but it worked well, and it should give a rough idea of what at least could be a possible build.

There are some fundamentals of deck building that everyone should know like deck thinning, searching, and card advantage. Just by understanding those 3 things can allow you to make decisions to what card to keep or not. Then there are things you learn after dueling for a long time. I would suggest to try to beat the AI decks as a measure to see what your base decks can do. The reason you find Malefic easier to use is because it is a beatdown deck. That is, there are no challenging combos in the deck. It involves only monsters with high atk and a few support cards. It is one of the easiest decks to use.

Vertex has several good points, so I will not repeat them. I will only state the ones that I agree with. Mound of the Bound Creator x 3 is a must in any Malefic deck. It's standard. Anyone that still uses Malefic World is outdated. You don't need any other field spell cards. Terraforming should be maxed. The only useful Malefic Cards are Malefic Cyber Dragon and Malefic Stardust Dragon. That's it. Cardcar D should also be maxed as it is your primary draw engine. All other cards mentioned are not necessary for main deck.

Your extra deck should not be made up of rank 8s. The combo is with Mound of the Bound and Malefic Cyber End. When you xyz, it has a rank and not a level, so you no longer get the protection from Mound of the Bound. The only safe xyz monsters are Number 81 and Felgrand if you do xyz. Also, you can use Malefic Parallel Gear with Stardust Dragon to make very useful level 10 synchro monsters that will also get the protection from Mound of the Bound which include Leo and Dragoyctus.

There's a lot of other things to say, but you have to understand the choices of cards before someone can just give you a deck. You can check out my Malefic deck in my link below to my decks.

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