With nearly 900 Pokemon having been introduced since Gen I, players have plenty of type combinations to choose from when building their teams. Some are incredibly strong, but not all of them are stellar. When a Pokemon takes on two types, it carries the resistances and weaknesses for both. Sometimes, they can cancel each other out — but if the types have a weaknesses in common, the Pokemon will take 4x the damage from those moves.
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This leads to some dual-types being incredibly fragile. Some of these Pokemon can still be useful in battle, but others don't even have good offensive coverage to make up for it. Regardless, players who use these combinations need to keep healing items on hand.
Rock/Fighting
This combo lands itself a spot as the best of the worst types. While it does have seven weaknesses (the most that any Pokemon can currently have), none of those weaknesses are 4x super effective. What's more, the combination of Rock and Fighting-type moves allow the legendary Terrakion — the only Pokemon with this typing — to deal super-effective damage to eight different types.
Still, those who choose to use Terrakion must be wary in battle. Among its many weaknesses are three out of the five most common Pokemon types: Water, Grass, and Psychic. Trainers should be very careful when choosing what opponents to fight with a Rock/Fighting type.
Grass/Dark
With a 4x weakness to Bug-types and six other weaknesses to boot, Pokemon like Nuzleaf and Cacturne have a lot of opponents to watch out for. What's more, some of these weaknesses are very common, such as Flying-type. Thankfully, other weaknesses are rarer, such as Ice and Fairy.
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Grass/Dark does have a fair number of resistances going for it, including the very common Water type, and is completely immune to Psychic attacks. Unfortunately, neither typing in this combo offers very much offensive advantage; Grass is resisted by many types, and few types are weak to either Grass or Dark attacks.
Rock/Dark
Tyranitar definitely looks dangerous, and its attacking power certainly isn't to be underestimated. However, like Rock/Fighting, its weaknesses are numbered and very common. Fighting-type attacks spell doom for this typing, with 4x effectiveness. It's weak to six other types as well, including Bug, Water, and Grass — all common in the Pokemon world.
As above, Dark typing gives this Pokemon an immunity to Psychic-type attacks. It also has good type coverage, making it a decent offensive choice. Defensively, though, Rock/Dark's weaknesses outnumber its strengths. Considering the frequency of types that can annihilate this Pokemon compared to ones it resists (such as Ghost and Dark), Tyranitar is easy to wipe out.
Psychic/Grass
Exeggutor knows some powerful moves, but there's no doubt that this type combination leaves it (and other Pokemon of this type) with certain disadvantages. For example, the Psychic/Grass combo has seven weaknesses, including a 4x weakness to Bug-types. Its weaknesses outnumber its resistances, and even outnumber the types that deal normal damage.
On the plus side, Psychic-type attacks easily can take care of Poison-types that threaten it. Other than that, though, this typing doesn't have much offensive advantage and can't easily counter its other weaknesses.
Ice/Grass
As powerful as Abomasnow can be in the right hands, there's no denying that it and its pre-evolved form are incredibly fragile. They're glass cannons, hitting hard but easily destroyed. With a 4x Fire-type weakness and six regular weaknesses (including the common Flying and Bug types), it's easy to take these Pokemon out.
Though Ice/Grass resists a few fairly common types (Water, Ground, Electric, and Grass), four resistances isn't a lot for a dual-type. What's more, the combination of Ice and Grass-type attacks only offers super-effective damage against four types. It's not enough of a trade-off for the number of weaknesses.
Rock/Ground
Though this typing does offer good offensive coverage through its attacks, it unfortunately has just as many defensive weaknesses. Rock/Ground Pokemon have not one but two 4x weaknesses, Grass and Water, both of which are incredibly common throughout the games. It's also weak to Ice, Steel, Ground, and Flying.
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Rock/Ground's immunity to Electric is one defensive plus, and it has a fair number of resistances — including a double resistance to Poison. Pokemon like Geodude might be handy when battling a Poison-type Gym, but they can be easily taken out with a single Water Pulse.
Ice/Rock
Ice types are among the most fragile in the series, and Rock-types have plenty of weaknesses of their own. With two overlapping weaknesses, Fighting and Steel, this is another type combo with not one but two types that are 4x effective against it, and four more weaknesses as well. What's more, it only has four resistances.
Thankfully, the combination of Ice and Rock-type moves allow Amaura and its evolution (the only Ice/Rock types) to take out a variety of opponents, including the Grass and Ground types that can spell their doom. However, they generally can't do much to counter their two biggest threats, Fighting and Steel.
Bug/Grass
Neither of these types are considered all that strong, and when they're combined, their failings only compound each other. Bug/Grass types have a 4x weakness to both Fire and Flying, and five regular weaknesses as well.
Their offensive type coverage is also poor. Bug and Grass each have seven different types that resist their attacks, and neither offers up any moves that can help counter this typing's weaknesses. Pokemon like Parasect might fare okay against a Water or Grass type, but in most situations, the Bug/Grass pairing is incredibly flimsy and has little advantage.