I don't think Sakura Taisen is particularly difficult when it comes
to fighting end-bosses, but it could be frustrating when some Hanagumi
members got fried by one of the bosses' "Sure-Kill" attacks and you have
to start the battle all over again ^_^ So when fighting the later end
bosses, make sure all members of your team have their health points and
special-attack points maxed before approaching the end-bosses.
Approach the end-boss together, and try to position your team members
in the following pattern within the time of one or two moves:
Oogami should be adjacent to Sakura (or whomever he can use combine-attack
with) initially so they can use their combine-attack. Afterwards move him next
to the end-boss for normal attacks.
There can be variations to this pattern but the main point is that
everyone should have the end-boss within their attack range (sure-kill
and otherwise) and that Iris should have everybody within her sure-heal range
(so that she can heal everyone after the end-boss' sure-kill attack).
Throw everything you got at the end-boss once in place, i.e. sure-kill
attacks should be used by everyone and combine-attacks should be used if
possible.
After that continue with normal attacks, and healing members with Iris'
magic if necessary (use Iris' magic wisely,as you may not get a second chance ^_^).
This worked for me with most stationary bosses
(finished Tenkai with just one attempt ^_^), but
could be tricky with the moving ones (need to corner them first, or alter
the pattern above to surround the end-boss completely).
Another thing to bear in mind is that
everytime you attack an end-boss, you'll increase its special-attack points,
so get ready to withdraw your team members if the end-boss' special-attack
points are about to max out and your team is in no condition to take the
wrath of its sure-kill attack.