"Have a grenade!" (said right before the speaker pulls the pin and throws it)
"Eat THIS!"
"Puny GOD."
And I'm looking a phrase that someone would say right before delivering a thorough beat-down...something like "I'm going to kick your ass so hard you'll be shitting boots for a week." The more vulgar, the better
I never expected this thread would require Finnish of all things, and so quickly... I'm no professional translator, but I'll give it a shot.
"Have a grenade!" = There's a few ways to do this. ''Otappa kranaatti!'' conveys a bit of a fed-up or irreverent tone. Could be said in a snarky, frustrated, or patronizing way (depending on context). If that fits the personality and the situation, it's your best bet. ''Ota kranaatti!'' is neutral, sounds like you're just asking someone to take a grenade, though. I can think of other variations but they might change it too much from the original.
"Eat THIS!" = ''Maista TÄTÄ!'' - it means 'Taste this!' but that's the Finnish equivalent.
"Puny GOD." = ''Mitätön JUMALA'' would be puny in the sense of insignificant, powerless. ''Heikko JUMALA'' would be 'weak GOD'. Is the context here insulting a god-like being or dismissing God/a god or what?
As for a vulgar insult, oh god, uh... ''Kick (someone's) ass'' has a Finnish counterpart in ''vetää turpaan'', which refers to a mouth instead of an ass (or rather the word for a horse's mouth or face or whatever). As such, an option could be:
(in colloquial speech) ''Vedän sua niin kovaa turpiin että paskot hampaitas viikon'', which essentially means ''I'll beat your face so hard you'll be shitting teeth for a week.''
That uses some colloquial word forms, the formal version would be: ''Vedän sinua niin kovaa turpaan että paskot hampaitasi viikon'', either way. It's uh, vulgar, and sort of the closest I could get to the original. I feel like Finnish threats tend to be short and simple, so this is a bit tricky. I guess that's true for any language if we're talking realistic threats (anger, limited time, and/or drunkenness don't tend to result in very poetic threats). Anything more detailed tends to sound silly when put in the mouth of real people... but fortunately we're not stuck dealing with realism!
I'll try to get alternatives if you like. A fellow Finn suggested ''Saat selkääs niin lujaa että lahot laudat laulaa," which is a delightfully quaint-sounding and alliterative expression that would translate as ''You'll get such a hard beating even the rotten planks will sing''. It might be, uh, a little too quaint, though.
. "eine passende Antwort zur Situation ist" - I don't know why German changes word order to SOV in dependent clauses, but it does. Also, stylistically maybe better "darstellt" instead of "ist". "Zu der" is not technically grammatically wrong, but will always be contracted to "zur". 
