You can only use the grapple hook, but on sky 0 you can use the shotgun for movement only. I have tested it out and it is possible to kill all enemies by sliding.

you don't even need the grapple hook or the shotgun

spiktllama, yes you do for sky 0 you need shotgun to get out of spawn

watch the gunless speedrun, they don't use the shotgun to get out of the spawn area

Yh u wait for enemy with boomer gun to blow u out of there

But I don’t get how they make the boomer guy see them it never works for me :(

EDIT: (just realised this is a rlly old post xD)

you have to jump in a way so that you can see the boomer guy (a head at least) so he will see you too and try to blow you up. and you can use it to jump high and kill the 3 guys on spawn. (hope that made sense, tell me if you need more help)

I just can’t get him to see me but I get the rest

you have to go over to the left and jump slightly off the platform, that way they barely see you

Yall still debating if its possible while xinn has done it like ages ago

I just did it and I somehow hate sky 0 even more

I dont know how you can move around that good in this game xD