The first order of business is to remove the Huge Slug's protective coating, otherwise your attacks will be cut down. A single Fire attack, or even just a Molotov can remove the coating, allowing maximum damage to be dealt. In general, Fire attacks work the best against Huge Slug, so emphasize on that. Don't use Accession, as it gives the Huge Slug more HP to steal through Siphon. Though you can use Accession with just the Flame Gene to form a Flame Whelp, so the HP gain isn't super high while still getting the benefits of dragon power. If so, then attack it with Flame Claw repeatedly. If not, then Ryu should just use physical attacks, buffing and healing when needed. Momo should use physical attacks, Might and Quake. If using Nina, have her use Simoon, Magic Ball, and/or Mind Sword for the most damage. If using Rei, you can use Weretiger here for heavy damage, though be sure to have someone use Influence to protect the rest of the party from berserk attacks. If using Garr, physical attacks plus Focus work well here, though he isn't highly recommended due to his high HP, which can boost Siphon's HP recovery, especially considering how often Huge Slug uses it. If using Peco, physical attacks are best, especially if he's equipped with the Flame Talons. If you want, steal the Vitamins early in the battle, though it's not a high priority. Beware of Quake, as it can deal some heavy damage to the team, which it'll start to use when it gets worn down, so be sure to heal when that happens.