Reducing the live metrics would mean either taking agents away from that EM or getting the agents to report fewer metrics.
Moving the agents would be OK if you have another EM they can go to, and you can restart the applications so that you can update the agent profile to point at a different EM.
For lowering the amount of live metrics
You could run a search on 'Metric Count' for agents on your EM and sort by agents with the highest value.
Then you have to go in one-by-one to your top offending agents and check their Metric Count pie chart and see what is the highest metric values.
Obvious ones are SQL, Sockets, JMX and PMI, but sometimes it can be things like JSPs, it will depend on the nature of the application.
Depending on which type of metrics are prevalent determines the solution to the issue, assuming that you will be able to reduce the amount of metrics without affecting dashboards, alerts, reporting etc.