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Personally, I'd try to warn my players that something very powerful comes their way. That could be giant footprints, the crushed remains of powerful other creatures or a closing thundering. So they might have the choice between fight and flight. Or at least the chance to prepare, both mentally and mechanically. And if they flee, still I would have established that really dangerous creatures roam here.
If they had real trouble with Huge+ creatures (which not necessarily equates very powerful), I'd give them some hints how to fight them. Reach weapons, specialized feats (Just out of Reach) and size increasing magic come to my mind.
Monsters killing average PCs with a single hit have their place, but I'd rather use them as story elements than as regular encounters. To sow despair when the party has to watch the inevitable destruction of a settlement, as bodyguards of creatures that are not supposed to be attacked (at least right now) or as threat that could be released if they don't act fast enough. And leveling / gearing up to be able to beat an once overwhelming foe is very satisfying, so I'd try to offer my players a rematch.
If other monsters don't really challenge them, more battlefield diversity (fog, lava, walls etc.), NPC magic usage (illusions, healing, slowing etc.) or special combat tricks (Pushing Assault, Improved Reposition, Spring Attack etc.) can make a difference.