Hints & Solutions Edit
Highlight the white text inside the block for hints ranging from vague to specific...
This puzzle is really hard, but rewarding, so do not despair or worry you are doing it wrong.
The instructions are a little confusing, so here is some clarification. For the first clue, the "(9 9)" means you are looking for two 9-letter words.
So, if you add up MALIIOSFAACIE you will see you only have 13 letters, so will be inserting five more to make 18 total. These five letters will come from the answer to one of the other clues.
Be very careful, it is easy to get confused between the answers to the clues and the inserted letters into the clues. They are distinct categories you need to keep straight!
I started with #15, which may also be a good starting point for you. Stare at the letters GIEFRMATION and try to deduce what two words would form from them.
The inserted letters from #15 are from the answer for #9.
Here are all the answers:
MALIIOSFAACIE -> maliCioUs faLLacieS (CULLS) = LIBEL
RCESAISTEPERO -> FOrces aRMistICe perIoD (FORMICID) = TRUCE
TERALLYANFU -> LIterally BanEfuL (LIBEL) = TOXIC
UNTSNMA -> Hunts AnImaLS (HAILS) = CULLS
INSETINTENTFAILY -> inseCt in tHe AntS faMily (CHASM) = FORMICID
DRANEDTEERTUEOU -> draIned teMPerAtuRe ouT (IMPART) = CHILLED
GULHTATSWA -> gulCh tHat Is waLLED (CHILLED) = CHASM
IINGULTHIER -> BiLinguAl thiNKer (BLANK) -> DIGLOT
IINGULTHIER -> WIne itaLIans ingEST (WILIEST) = VINO
NEITAANSING -> BegUILing the mosT (BUILT) = WILIEST
WHIEBNROSSWORDS -> whiTe bOX In Crosswords (TOXIC) = BLANK
CONSTD -> consTRUCtEd (TRUCE) = BUILT
ESNATIONAPPIEDTOHERS -> DesIGnation appLied to OThers (DIGLOT) = THEM
AHEEMTRIZATIONAEW -> aChIeVe mILItArization aNew (CIVILIAN) = REARM
GIEFRMATION -> giVe INfOrmation (VINO) = IMPART
COSOWNSAPETNGORM -> coMEs Down As a peLtIng STorm (MEDALIST) = HAILS
NOINTARY -> noT in tHE arMy (THEM) = CIVILIAN
PIZWEINGAN -> pRizE-weARing Man (REARM) = MEDALIST
The shaded letters spell LETS REMOVE NUMERALS.
As suggested by the use of "Pluto" and "legionnaires", the Shades are Roman.
If you remove the Roman numerals from all the answers, you are left with:
BE TRUE TO US FOR HE HAS GOT NO WEST BANK BUT THE REAR PART HAS AN EAST
The monster with "EAST" as a 'rear part' is the NETHER BEAST.