I have a lvl 50 Invuln/Fire Melee tank and would like to know why I can run Council and DE @ 4x8 with no trouble at all but instantly get stomped into the ground by groups like Longbow, Carnies and Arachnos when running tip missions at the same level. I went broke crafting this build (easily over a billion) and needless to say I'm extremely pissed....

Anyway, any help or advice as to what I'm doing wrong will be greatly appreciated. Here is the build...
|MxDz;1248;648;1296;HEX;|
|789C5D93DB4E13511885770F30505AA12DD072AA057AA02D3 3D0186F3CC688241888|
|4D20DED652B674621D9A6931D62B9FC0E80B68D437F27C7A0 50FF72AF8CF5A14D19D|
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|EA7AA4E2EEDDD59D86DE6FE7D3ABFB6ED7504AA558535DD1B 7B5D3D6D69A736FBFE9|
|68B7B66D37ED4E377994DED04DADAD555BBB5D3E47D79C867 6B5D3B17A0FA1CADE5E|
|D3DA6C69BD13C6E3BAAEB56C6777B8F7B2A3DD76C36E45F0B E6AEF363A928D5F6BD9|
|75EB8A5BEFD8F5EA46ADDD911D92E2EBB4FC5E0DAAA3D5A70 A3EA5F2CA9F10CC287F|
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|4333D00E513B4CEDB0099CA2763CE4A9A229F0D38FC42F60E 437E165CB2A76206FA6|
|8A1D22386778FEBDE5EF2B61EFD17FF0469C18C8078D4BB03 07E91B80024CF13E780|
|B752CDB10406E97B7281C800D379414E8D8AD3218E61A8051 BEFA42B421B11364EB3|
|319503C6C5C3303B8667612C2191283BA29FA091FA4C7C01D E8B629C56E2544C5331|
|4D2BE92C85C5CA18CF37F6148DB3CF88E7C40B60FE25F0414 413144D8C61E8F37122|
|0664A2C047299B60D904F39951CC2733804D737D4042F69EE 2079BEAE250B907043F|
|5FDEFB6E96CAF3C0F90304A386278435C384DFE085F4EEE5A 12C1590C01CB79F3B8B|
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|043CA358CB38CC74501D2F0496FF0FAC9F08F463469560EFE FA37C69C87E0DFD8D98|
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|A37B|