Against suit contracts-you do not have an attractive opening lead-no singleton-no top of a sequence -no partner's bid suit.The best thing to do in these situations is to eliminate the NO-NO's ,and then choose what is left.
Bottom from a heap is a complete no-no. as well as underleading away from an Ace ! Do not lead from a suit which only has an Ace unless the Ace is from Ace/ King at trick one.
Let us look at these situations and listen to the bidding to guide us on what to lead against suit contracts