Old deck, but wanted to keep it on here because it's a deck I playtested constantly and think it's good in this form. Here's the updated list - https://digimoncard.io/deck/megagargomon-budget-updated-for-bt20-118068
This is a cost effective green control deck that has a ton of play options and potential combos. Rapidmon X was deliberatively left out to keep costs low.
Deck Strategy -
The three important aspects of this deck are - suspension, piercing, and alliance. You can pull off a few combos that can check security 3-4 times in one turn while wiping your opponent's board via combat. The wincon is to get MegaGargomon ACE out quickly and use alliance, piercing, deleting and security card trashing to widdle security down as fast as possible.
Good starting hand - A tamer (preferrably the memory setter Henry Wong), a terriermon, a lvl 4, a lvl 5, and a MegaGargomon ACE. If you don't have these pieces, try to search for them as quickly as possible.
First turn - build out a terriermon in your breeding and play a tamer. Tamers are extremely important to Rapidmon's effects, so you want to play a tamer as early as possible, even if you need to take the memory hit.
Second turn - Make sure you have your MegaGargomon ACE at this point, if you don't, search for it. Sometimes, this isn't possible. For searching, you can play double typhoon and then follow up with the terriermon searcher the next turn. this will sift through 7 cards total. I often digivolve terriermon then terriermon X in breeding to grab 2 cards for free early game. That helps and it gives piercing to the stack.
Regardless, you want to throw out a rapidmon by turn 2 (preferably the blocker rapidmon to slow the game down.)
Control the game as much as possible to get your pieces out. You can use Leopardmon X to get bodies out and to bounce their digimon back to the hand. Leopardmon X isn't the wincon, just a tool to slow the game down and get MegaGargomon ACE out. Leopardmon X can also be used to play terriermon/lopmon out for free, so it accelerates the game.
Midgame and controlling the board - Ideally, you'll want to counter digivolve to MegaGargomon ACE on top of ST17 level 5 Rapidmon as fast as possible. With the ST17 level 5 Rapidmon's inheritable, you can have them trash a security card for a digimon you delete in combat, so counter then block for one security check. You can even hard play the rapidmon for a cost of 7 to do some removable (usually your opponent's ACE, so you can get the overflow bonus.) But, don't be shy with MegaGargomon ACE. Digivolve to it often regardless of overflow. This allows you to use MegaGargomon ACE's digivolution ability to stun their lvl 5s and 6s so they can't ACE Digivolve when you attack.
While you're controlling the board, get out Willis so you can have the benefit of costing down your digivolutions. You can also use hidden potential discovered to get your ACE out for free.
Endgame - Hopefully, by now, you should have a lot of combo options. Alliance is potent in this deck, so hopefully you can get a lopmon out around the endgame. A good example of a combo would be to swing with MegaGargomon ACE (with ST17 level 5 rapidmon and terriermon X in the digivolution cards), delete a digimon with piercing, unsuspend, then have lopmon give MegaGargomon ACE alliance, swing again into security for 2 checks. It's better if you can have two lopmons out so you can double up the alliance and do 5 total security in one turn. This is where this deck has a potential weakness. It has problems getting those last one or two hits in. So, be mindful of this and flood the board with rookies if possible.
The Henry Wong memory setter is important for suspending one of your opponent's digimon when you swing with your digimon. Try to swing with a lower level, like terriermon or rapidmon, use Henry's effect, then attack with MegaGargomon. Depending on your level 5, this should either be a piercing attack or an attack that will make your opponent trash the top security card.
If an opponent is making a comeback or is harder to take down by one off security attacks, Quartzmon is a good endgame piece for slowing the game even further. It's difficult to search for, so this usually is available toward the end of the game when you're close to decking out. Don't rely on Quartzmon, it's a good comeback strategy or last resort.
Tech choices for this deck - Shoto for piercing / blocking at the end of your turn for even more control. Shoto memory setter to have a better chance at memory setting, and his ability could be used early game to knock out some unsuspended level 3s. Combine both Shotos for a really easy early game check. One tech suggestion would be to remove Giant Missile for Heaven's Judgement. You could also slot Mimi in or more searchers like Agility Training. You may consider swapping out the level 4 gargomon. Their jamming effect isn't as useful but they were kept in for their inheritable. Maybe replace them with 2 agility training for searching.
And that's it, comment down below for suggestions.