Tribute Summons can be dangerous surprising as any monster summoned can be tribute-doll into the dragon. While you are safe for 1 turn, you need a 2.4k+ Beater to overcome this problem. But be aware, that one of his monster can lock on your beater to destroy it after the third turn. But if you have enough ATK the turn after its summoned, you can OTK him through this, else you have either to wait or tribute your monster to remove the indestructible effect on that monster(or use piercing damage) or a destruction effect on it.
Elements unite skill is very useful and can be used against Union, Tribute, Fusion, Cu and Energy. It can't be used against Spirit, Charmer and Thousand Dragon as it's a gamble you very likely lose against, as those have one or several ways to either get rid of the Gate Guardian or damage you directly. Like the Rabbit for Spirit, Time Wizard for Dragon or any attribute Charmer stealing your Guardian.
If gamea/someone wants to know strategies against the lvl 20 SDs.
Bring 2k+ ATK/Def monster against Cu and you easy win.
The Charmer deck is difficult without any protection against their monster effects. If it sets a card, then you need 2k+ atk to overcome the charmer else you risk the no damage. W/o any backup it's possible that your own strong creatures will be used against you. To counter this bring either strong monster effect-immun monsters or use the ritual cage bringing skill with ritual monster use from Alexis(I do it so).
Fusion combination can bring out a 2200 atk monster(against monsters), so you need something that is even stronger ATK/Def-wise. Else anything over 1.5k will easy wreak this deck.
Gemini Monsters are easy to beat with 1.6k+ ATK monster.
For Spirit Monsters you need 1.8k beater to overcome even the decent Maharaghi. Stalling can be dangerous with the burn spell+Inaba monster.
Thousand Dragon is really annoying, not only with the Time Wizard, but also the fusion Thousand Dragon that can overcome normal strong beaters. So you need at the same time high ATK/Def and protection against monster effects. I use ritual monster with the ritual cage skill from Alexis for this cancerour deck.
Thousand Energy can be surprising with its instant high ATK monster spawn. Anything below 2.4k ATK/Def is in danger except if it clog its field with the Balls. Then you can stall until you can bring the big beater as it will normally not use more than 1 energy(but I had the case with 2 played energies).
Union Aid can be very luck dependent/dangerous due the Flamberge sword. If it goes first, it can bring out a 2k beater, but the worst is when it goes second and have 2 of the swords + beast pumping out a 2.5k killer. Any destruction effects(that are not monster effects as those get negated at destruction) are very recommended in combination with 2k+ ATK/Def monsters.
My lvl 20 auto deck. Only pain is viser des. If he shows up, just tribe that monster and run him over. Otherwise an easy deck with Yugi. I love arsenal with element doom as a team. Most of my quickest wins came from these two.
Yugi with the Power Of Dark ability is your best bet to powerhouse through the early game. Most packs can net you 1500 Fiend or Spellcaster beaters to pump up to 1700 with your Yami field. Flame of the Tyrant has Frontier Wiseman and Emissary of the Afterlife which both become 1800 beaters with Yami. It's also a good pack to invest in if you want to build a strong Fire deck.