Gliscor has just the right base stats, movepool, and typing for what it needs to do: counter many physical attackers and support the team at the same time. Its STAB Earthquake 2HKOes Tyranitar and OHKOes Lucario, and also does a decent amount to most fragile switch-ins. The instant recovery provided by Roost is of great use to a Pokémon with Gliscor's bulk and relatively high Speed, and helps keep it alive for the entire duration of the battle. It also allows Gliscor to shed its Flying-type for a turn, giving it a valuable resistance to Rock, which can be used to stall out a Choice Band Tyranitar's Stone Edge quite easily. However, be careful about Roosting against Fighting-types, as Close Combat can do a big chunk to Gliscor while it doesn't resist it.
Gliscor gets plenty of opportunities to support the team by setting up Stealth Rock, and as such, is one of the better users of the move. However, if you have another Pokémon to set up Stealth Rock elsewhere in your team, Knock Off can be of great use as well, in helping to deal with sweepers that rely on their items to do significant amounts of damage. For example, without a boosting item, Tyranitar's Crunch will only manage a maximum of 36% against Gliscor, and so Gliscor can come into a Crunch, Knock Off the Choice Band, and then survive to Roost up to greater health. Similarly, Scizor's Bullet Punch doesn't hurt nearly as much without a Choice Band or a Life Orb boosting it, dealing only 24% damage on average. Stone Edge and Aerial Ace are both options for use as a secondary attack alongside Earthquake. Stone Edge is generally the better option, as it gets better coverage with Earthquake than Aerial Ace does, and can be used to get a solid hit on Gyarados, Zapdos, and Salamence switch-ins. Aerial Ace can be used to specifically deal with Breloom and Heracross, both of whom resist Earthquake, and also allows Gliscor to get some damage on Celebi.
216 Speed EVs allow Gliscor to outspeed all non-Choice Scarf Heatran and neutral natured Lucario. This is of particular importance when trying to counter the latter, as Lucario may carry Ice Punch to deal with Gliscor. Jolly Lucario still outspeeds Gliscor, but Adamant is usually the nature of choice for the Swords Dancer, so unless the rest of the team has severe difficulty with Lucario, Impish Gliscor usually suffices.
Most Water-types partner up well with Gliscor, providing resistances to its Water- and Ice-type weaknesses, while also drawing in Electric-type attacks to give it free switch ins. Vaporeon has a high Special Defense stat to make up for Gliscor's lackluster Special Defense, and it helps deal with the only Fighting-type that Gliscor cannot take on reliably - Infernape. Suicune can make the most of the Stealth Rock laid by Gliscor by utilizing Roar to shuffle the opposing team around and spread residual damage. While Tyranitar doesn't resist Ice and shares a common Water weakness with Gliscor, it is naturally very bulky and its sandstorm bolstered Special Defense can take weak special attacks without too much trouble. More important is the evasion boost Tyranitar's Sand Stream provides for Gliscor, which also helps by wearing down many of Gliscor's common switch-ins.