Radagon is a great fight, but a good boss is not always a good co-op boss. Radagon drowns out all other summon locations because all the bleed build losers can't cheese him to death. Disabling this pool will guarantee a better Elden Ring co-op experience.

Step 1. Disable the anti-cheat (use google)
Step 1. Open ER TGA cheat table (use google)
Step 2. Attach to Elden Ring and open table
Step 3. Click the Event Flags square
Step 4. Find "Unlock all Summoning Pools", go to the right of it and double click <script>
Step 5. CTRL + A and delete the entire script
Step 6. Paste the below script

{$lua}
if syntaxcheck then return end

[ENABLE]
local flagsBase = {
19000040, --
}

local flags = {flagsBase}
--[[if readByte(readQword("CSDlc")+0x11) == 1 then table.insert(flags, flagsDLC1) end
if readByte(readQword("CSDlc")+0x12) == 1 then table.insert(flags, flagsDLC2) end]]

ef.batchSetFlags(flags, 0, "SummoningPoolsThread")
disableMemrec(memrec, function() return not ef.SummoningPoolsThread end)

[DISABLE]
ef.SummoningPoolsThread = false

Step 7. Run script
There's probably an easier way to do this but it works and has saved me from quitting the game