You will need to have another character (on a different account) pull the missions from this agent, and then turn in the completed missions while in a fleet with your character who has bad standings. If you do not have 2 omega accounts to do this with, you will need the help of another player. While not a standard service they offer, I believe the USIA program can assist you with agent-specific standing correction. You can find their services thread here: 
We Raise NPC Standings: Lower Broker/RELIST Fees ★ Access Epic Arcs + Mission/R&D Agents ★ And More!
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