This set turns Regice into a good special sponge capable of taking on a variety of special attackers, crippling them with status and wearing them down with Regice's decent offenses. Thunder Wave slows down incoming physical attackers, while Toxic helps wear down bulky opponents, especially Water-types such as Milotic and Slowbro. Ice Beam is used for STAB and comes off of Regice's useful base 100 Special Attack. Thunderbolt and Charge Beam provide excellent type coverage alongside Ice Beam, and Seismic Toss provides consistent damage.
A Calm nature along with 252 Defense EVs make Regice extremely bulky, taking both physical and special attacks with relative ease. If you desire a more specially defensive spread, you may shift the Defense EVs into Special Defense, though you could also run a Bold nature for a more physically defensive set. Rest can be used in the last slot as it rids Regice of harmful status and fully restores its health; however, when Regice is using Rest, a cleric such as Clefable is recommended as Regice becomes setup fodder for numerous Pokemon. Explosion may seem unorthodox on a defensive set, but it deserves a mention because it allows Regice to blow up Fighting- and Fire-types such as Blaziken and Hariyama.
As this set lacks reliable recovery, Regice appreciates Wish support. There are a variety of Pokemon that can provide this, namely Clefable, Leafeon, and Chansey, all of which have good defensive stats and access to useful support options of their own. Due to Regice's mediocre typing, you'll need Pokemon that can switch into Fire-, Rock-, Steel-, and Fighting-type attacks. Claydol resists Rock- and Fighting-type attacks, Altaria resists Fire- and Fighting-type attacks, and Moltres resists Fire-, Fighting-, and Steel-type attacks. There are many more teammates to consider, though, as these are just a few examples. Regice is also weak to Stealth Rock, and the only way to remove it is by using a good Rapid Spin user. Claydol and Donphan both resist Rock-type attacks and have access to Stealth Rock themselves, while Blastoise can switch into Fire- and Steel-type attacks with ease.
Pokemon that can take advantage of Regice's stalling ability are good partners, particularly those with low Special Defense. As for offensive partners, Aggron can take advantage of Regice's high Special Defense and ability to switch into Water- and Ground-type attacks, while it can set up Rock Polish to pummel the opponent and appreciates the paralysis support Regice can provide against faster sweepers. Blaziken appreciates Regice as a switch-in to Water- and Psychic-types such as Slowbro, Milotic, and Alakazam, and Venusaur appreciates Alakazam taken care of as well. As for defensive partners, Weezing is an excellent choice because it only has one weakness, and Regice can switch into most common Psychic-type attacks with ease. Donphan is another good partner because Regice can take on the Water-, Ice-, and Grass-type attacks aimed at it, while Donphan can remove Stealth Rock and switch into Rock-type attacks.