It's easily possible, and it becomes even more so when you unlock the ability to get Wentworth to organise your troops, and to spy on which troops your opponents will use first.
I beat an army more than twice the size of mine. It's all about utilising the features available to you.
Ah... I believe I'm using those features already. Those are the options that costs some coins right? (Playing on JP 3DS and can't read Japanese lol) Will have to try engaging them even if I have fewer troops then.
Also, I finally got all the diamonds on all stages for Squad. Just missing the target score and no deaths achievement for 5-3 then I'm done. Almost finished arcade mode as well but died on 5-2. Hopefully, will be able to finish it on my next attempt.
Btw, what's your latest redeemed hat/outfit? Just got my tanooki (or is it fox?) Luigi set. Really cute.