The Welter summon some special hydras called Spawn of the Welter. Those hydras use some attacks that heal each other, so the best way to kill the boss is to not let the spawn of the welter be nearby it.
There are 2 ways of doing it: the first way is to lure the spawn of the welter away, kill them and after that start killing the welter.
The other way is to kill the eggs that are summoned by the welter (if you don't kill them they turn into the spawn of the welter) before they become the spawn altogether with killing the boss.
If you do that, it won't be that hard to kill the boss, just treat it like a normal (but stronger) hydra