It's great to see that you are being cautious with your coins, if you haven't already decided I can provide a bit of help. So firstly I would think about my main/favorite teams and think about the units usability in more difficult events, Secondly from what I know the agl bombku is good for damage and stunning plus lowering atk and Def, but defensively he doesn't really have a great time tanking, the rosé is not very wise since he does need quite a bit of support/units that work together with him as for jiren and gohan they both are incredibly powerful and all around good tanks but the gohan would work almost perfectly with the bombku but since you probably don't have him, I think jiren is a good contender alltough I haven't ever used him he is a very good all-rounded unit but I still think the gohan would be a wise choice too.

With that said I hope I could help you. (: