This question is rather ambiguous. If you’re attacking with aircraft only (no land units), it’s not an amphibious assault. If you mean, “Can I conduct an amphibious assault without cruisers or battleships for shore bombardment?”, the answer is yes, you can. When conducting an amphibious assault, you have to choose which aircraft will participate in any naval battle and which will participate in the land combat; they cannot do both. Any naval battle must be resolved before amphibious landings can take place.