Jotun Grunt and Grim Lavamancer

[Scordata]

This problem came up last night, and we were racking our brains, but couldn't figure it out.

At the begining of my upkeep, I add the third cumulative upkeep counter to Jotun Grunt, and proceed to name cards for each counter. For the first two counters, I choose 4 cards from my graveyard, but for the remaining counter I choose 2 cards in opponents graveyard. Opponent then activates Grim Lavamancer's ability, removing the chosen cards from his graveyard.

What happens to Grunt? Am I forced to sacrifice him because I can't pay the cost? Am I allowed to choose new targets? Can My opponent even do this? If so, when?

Thanks.

[luma]

Jötun Grunt doesn't target the cards it removes (it says "remove two cards in a single graveyard from the game"). The cards are chosen when the ability resolves, and at that point it's too late to respond.

[Julian23]

The card's are chosen on resolution and put on the bottom of respective libraries without any player receiving priority; there's no targeting involved. However, your opponent can activate his Grim Lavamancer and remove 2 of his own cards before Jotun Grunt's CU trigger resolves which might make you sacrifice it in case you can't remove enough cards.

[IsThisACatInAHat?]

Is it possible/ legal to partially resolve the trigger? For example, it's your upkeep and you control a Grunt with 3 age counters on it. If you have 0 cards in your graveyard and your opponent has 4, can you put the 4 in their 'yard under and then sacrifice the Grunt as a result of not being able to pay the full cost of the upkeep on it?

[TeKo]

Nope all or nothing.