A single fear...
Link Nukem is a fearless warrior strong enough to conquer the entire universe if he really wanted to, but there is one thing that scares him. It is the thought alone that Weegee's inner self might break free from the seal. That is the only thing that makes him tremble with fear. TBWH, the strongest creature in the world and maybe even in the entire Multiverse would easily get rid of everything. Link Nukem holds dear and worked hardly to achieve for all those years.
If Link Nukem was forced to fight against his arch-nemesis, then even after gathering all of his strength his power level would only be equal to 4% of The Big WeeGee Head's power. That's why he's Link Nukem's only fear.
How to break The Big WeeGee Head seal.
If anyone besides Link Nukem tries to destroy the seal by attacking it directly, it won't work.
The seal's breaking down on its own is almost impossible, unless Link Nukem ignores it for a "really long time" or decides to break it by himself with a special technique (which he would never do). As Link Nukem is the one keeping the seal intact, killing him by one of TBWH's minions: Weegee, Malleo or Yushee would result in seal's destruction. The minion must kill Link Nukem by his own hands instantly. If he dies later from the injuries, it won't affect the shield and Link Nukem will simply revive like if nothing happened at all.