First, if you haven’t done so make sure you have Diplomacy skill trained up to level 5.
Diplomacy = + 4% Modifier per level to effective standing towards hostile Agents. Not cumulative with Connections or Criminal Connections.
After training that hopefully your effective standing with the agent will be above -2.00 and you can continue doing missions. When you have positive standing with the agent, make sure you have Connections skill trained up to level 5.
Connections = Skill at interacting with friendly NPCs. + 4% Modifier to effective standing from friendly NPC Corporations and Factions per level. Not cumulative with Diplomacy or Criminal Connections.
Also to increase standing gains for completing missions, make sure you have Social skill trained up to level 5.
Social = Skill at social interaction. + 5% bonus per level to NPC agent, corporation and faction standing increase.