A lot of times, those who battle a lot in Pokemon will talk about ‘coverage’. This is an essential part of battling, and one that has both regular and unusual ways of being achieved.
Coverage is what you have when a pokemon can hit most or all other pokemon for at least neutral damage- damage that is neither super-effective nor ineffective. While it’s most important for a sweeper, who simply cannot do their job without decent (if not perfect) coverage, it’s also important to be aware of on tanks as well. Walls generally don’t care as much about coverage- they like to use set damage (Night Shade and Seismic Toss) or damaging side-effects (Leech Seed, Poison, Bad Poison, Burn, Sandstorm, Hail, Rocky Helmet, entry hazards) to grind away an opponent’s health while they ‘sit pretty’, and those tricksters who like to deal damage do the same.
Anything that wants to be dishing out attacks, though, needs coverage. Otherwise you’ll find yourself swapping pokemon more often than attacking, which is no way to win a battle.
There are a number of ways to achieve coverage, and they range from the common to the bizarre.
Boltbeam is the most common coverage- while it’s usually achieved through the use of Thunderbolt and Ice Beam (hence the name), this term really refers to the coverage achieved by combinding Electric and Ice attacks. Very few things have the right setup to resist this set of attacks; Electric is resisted only by Ground and Grass, and Ice is resisted only by Fire, Water, and Steel. Not only does this make it impossible for a single-type pokemon to resist this combination, it pushes things further- Ground and Grass are both weak to Ice, and while Fire and Steel are neutral to Electric attacks, Water is weak to them- so the only ways to completely avert the power of Boltbeam are a special immunity (Shedinja, who is immune to both attack types thanks to its Ability) or being type-neutral and a massive special wall (Blissey/Chansey, Ferrothorn, or, in a pinch, something like Hypno).
Other combinations are also possible.
Fire/Rock/Steel is a good example. Fire is resisted by Water, Fire, and Rock. Steel is resisted by Water, Fire, Steel, and Fighting. Electric is resisted by Ground and Grass. Because of this odd setup, which does rely on eating three moveslots to function, some pokemon that are Boltbeam capable can instead pack a grouping that is supereffective in completely different situations. This particular combination requires Camerupt to resist it- and Camerupt hasn’t got the best defenses anyways. It also packs a decent array of side-effects- Fire inflicts burns, Electric lays out paralysis, and Steel has an array of self-buffs, debuffs for the opponent’s defense, and in Flash Cannon and Mirror Shot, a shot at killing their accuracy or Special Defense.
Rock/Ground is a decent choice as well- only Levitating things that resist Rock have any protection here, basically meaning Lunatone and Solrock, which aren’t the most common opponents, and Magneton, which you will likely want to use a Special Wall against anyways. Some walls like Forretress or the terrifying Ferrothorn will still give you trouble- so it’s far from a perfect coverage- but it’s still pretty good, especially given Earthquake and the high attack power of many Rock moves.
Poison/Ground/Electric offers interesting options for those pokemon that can’t wield Ice moves, since only Gligar will give it any real trouble, and putting the powerful Ground attacks together with the debilitating effects of Poison and Electric moves is pretty potent.
Dark/Psychic/Fire can work, since the only real weakness of Dark/Psychic is Steel types. This combination can be a bit difficult to pull together though, since many pokemon that can wield Dark and Psychic together aren’t terribly capable with Fire attacks. It isn’t hard to drop Fire and just make sure another team member is prepared for Steel types.
For instance, by wielding Fighting and Psychic. While it’s mostly limited to certain Normal-types and the few Fighting/Psychic typed pokemon, a pairing of Cross Chop and Psychic will critically wound if not outright knock out nearly anything- Darn Shedinja- if backed with enough power.
There are many other type-combinations for movesets out there- by considering what each type is poor at hitting and putting it together with something that can still hurt that or, better yet, will direly wound it, you can make sure that your pokemon are always ready to dish out hurt.