Marshadow : full-run carry guide
Overview
Marshadow can easely carry the whole run. The combination of fantastic typing, excellent stat spread, incredible moveset and passive/ability that superboost them makes it one of the best options for it. Marshadow is already strong without all of his aresnal, but with everything unlock, it's stupidly powerful.
Strengths
As a fabulous, its has the usual 600 base stats, that are incredibly well distributed, granting it high Atk, very high Speed and pretty decent bulk. Its marvelous typing, Ghost and Fighting, let it hit every single pokemon (but one family) with at least neutral damage with its STABs, letting it have 2 more moves to runs, granting it incredible versatility. Finally, its passive and ability works very well with the moves it can learn.
Weaknesses
Marshadow doesn't have any major weaknesses, except maybe its cost of 8 (6 when reduced), greatly limiting the pokemons you can take aside. With all of its tools, I'll say the only difficulties are bulky fairy types, wave 145 and 195 rival, and Eternatus. Its typing isn't the best against some tough points in the run, notably the rival's Rayquaza. Some end biome bosses can also be a bit tough to pass if you haven't done some Power-up Punches before them. Overall, its strengths greatly overshadows its weaknesses, having various ways to bypass them.
Recommended build
Technician in ability (not like we have a lot of choices..), it's fast enough to run the Adamant nature. For the items, with the exceptions of SpA boosting ones, every stat boosting items (protein, iron..), and every berry that you will find, type boosting items are also very strong (especially Black Belt), and every ultra and rogue item that can be equipped.
Moves & abilities
Its moveset consists in two things : its STABs and its coverage. For the STABs, i'll strongly recommand Power-up Punch and Spectral Thief. As for the coverage, it has a lot of choices, like the elemental punches, my personal choice would be its two eggs moves Bonemerang and Triple Axel. (I'll explain everything down below) Power-up Punch is, for me, its best move. 40 power doesn't seems much, but it's boosted by STAB, technician and Iron fist, so it will hit hard. Boosting Atk for each use is THE thing that makes Marshadow so strong. After 2 uses, it will practically one shot everything. If you find the Multi Lens, pick it, the move will now give +2 Atk each turn. It's the move that you will be the more likely to click. Its Ghost STAB will evolve thought the run. Starting with Shadow Sneak is already good, boosted by technician and being a priority move (even thought Marshadow will probably outspeed everyone, especially consider no pokemon is evolved in the first waves). It will quickly learn Shadow Punch, and later learn Spectral Thief. Each move have their advantages, but I'll pick Spectral thief as the better option. Shadow punch hits harder and has more PPs, but stealing stat boosts is far much useful, especially against bosses with a lot of berries, and is a crucial tool against Eternatus. Triple Axel is its first coverage move. The primary reason to use this move is to pass rival's Rayquaza, and flying pokemons. With Technician, it reaches stellar 180 base Power, and since it's a multi hit, it's working great against bosses. Be careful with its precision, some wide lens will never go amiss. Bonemerang is its second coverage move. Its main usage is to pass Rock and Steel pokemon with Sturdy, since it's a multi hit (Here again boosted by Technician, for incredible 150 base power). It's probably its less useful move, but still does a very good job at what it's meant to do.
Strategy
Marshadow will be played roughly the same during the entire run, except for the final boss. It has the advantage of having its full moveset available at the start of the run. The only interesting moves that it will learn later are better ghost moves, and Drain Punch and Close Combat (that I don't use). So the start moveset I recommand is Power-Up Punch, Shadow Sneak, Bonemerang and Triple Axel. If you want to use other moves (or if you don't have the recommanded egg moves), there are plenty of options. As I said earlier, Shadow punch can be run instead of Spectral Thief for more raw power (But change it to spectral thief before Eternatus). Thunder Punch is a very solid move (that I have used myself before opting for Bonemerang), it's very strong if you choose to pass thought a lot of water biomes. Drain punch is also a very good option, greatly reinforcing Marshadow's survivability (don't replace Power-up Punch with it, just remove Bonemerang). The other options are pretty niche, or simply outclassed by other options : its last egg move Meteor Mash is only useful against Fairy types, Ice punch is a worse Triple Axel and Fire Punch hits almost nothing that Marshadow can't with its other moves. There isn't any complex strategy for the run. Just use Power-up Punch at the start of every biome and trainer fight (if the opposite mon isn't dangerous). The only fights that will require thinking will be the major boss fights : wave 115 team leader, wave 145 rival, wave 165 team leader, biome 180 council 4 and champion, wave 195 rival and wave 200 Eternatus. The fights will depend on the pokemon that your adversaries will have (except Eternatus). But overall, Marshadow should be strong enough for you to be too worried about it. for the team leader, use Power-up Punch in priority. For the council and champion, heal well Marshadow before each fight, and be careful with its PPs, especially if it have no leppa berry. For the rival, since the starter will be terastallize, Power-up Punch will always be effective against it. If they switch, they'll probably send a flying pokemon, but since all of them except one have a secondary type weak to fighting this will not make a difference, although Toucannon can be annoying since it can burn Marshadow. If they have a ghost type, use your ghost move, it should be enough to One shot it. Against Rayquaza, Triple Axel. Globally, Marshadow has a response against every pokemon. Against Eternatus, if you have enough items like Soul dew and proteins, Power-up Punch should be able to take down one bar. If that's the case, use it until Eternatus reach phase 2, and then use Spectral thief, then use at will Triple Axel, or Spectral thief again. If that's not, use triple axel after one bar goes down.
Synergies
Marshadow can go well with a lot of pokemons. Being a solo carry, the best thing to do is putting low-costing shinies, to boost the shop as much as you can. If you have a pokemon with the pokerus, it's also a very good boost. Pokemon with item harvest abilities (Magician, pickup..) will also be great supports. And finally, any pokemon with Revival Blessing, in case something unexpected happened and Marshadow gets KOed. If you're afraid Marshadow cannot take down Eternatus by itself, choose a mon specially for this.
Tips for beginners
If you haven't unlock the egg moves, use the elemental punches. They're still powerfuls and provide a lot of coverage. If you're afraid Power-up punch will not do enough damage, just click the most effective move. Marshadow is strong enough.
Tips for advanced players
Marshadow is pretty easy to play, so there isn't really some complex strategy to do with it that I'll recommand. If you have the will, after waves that you have passed quickly (like one shot a pokemon), you can reroll the shop until you have no money, and then reload the page. This way, you can see what's there in the shop without consuming your money.