Warmaiden's value is that her decrease defense is AOE - Ninja's is only single target. Your replacement of Warmaiden isn't Ninja, but Spider. Spider has quite a nice kit, great aura too, but his main downside is the sheer number of books he needs - and even after all those books, he still has a worse cooldown on the AOE decrease def than Warmaiden, though overall he's much better of course. So how many epic books do you have that you could put on him, or is he already booked? If you have the books, I'd recommend Spider as your next 60. If not, it's a harder choice. 

Another champion that you should start working on is Quargan, he's quite good. Your main dungeon team should probably be Spider, Kael, Ninja, Apo and Quargan - at least for the basic dragon / ice golem progress team, dungeons like FK or Spider tend to need more specialized teams. However, your Kael is going to need accuracy, because with just 28 accuracy, he'll put very few if any poisons, losing most of his utility (at least on bosses; on waves, his AOE damage obviously makes him useful). 

Basileus is indeed not a great legendary, but he's still a legendary and has an AOE stun ability, so can still be useful in some places for sure. 

I would take Spider to 60 next. A great champ worth your epic books.

I don't find warmaiden as effective because I already have decrease def
on ninja, but I've been advised otherwise. Am I missing something with
her? 

You don't need to rank up Warmaiden further if you go with Spider and book him to increase the chance for decrease def to 100%. He is a better version of Warmaiden, as he does the same she can do + additional chance for weaken and + decrease attack on the A2. 

Ninja is no replacement for Warmaiden/Spider, because his decrease def is only single target, while Warmaiden/Spider put decrease def on all enemies. You should tune the speed of your champs with speed stats from gear, so they act in the right order. If a champ like Warmaiden/Spider goes before your Kael, you can put decrease def on all enemies and then attack all enemies with Kael's strong hit. That doesn't work with Ninja, who can put decrease def only on one enemy. 

I also vote for spider. Awesome dungeon aura. Perfect debuffer for arena as well. The long cooldowns are a non issue in all dungeons and arena nuke team. 

I was thinking Spider was the best option but been told he won't be useful for a while. I don't have the books to fully book him either. Still worth taking him to 60 or slowly work him down the track? I will take Quargan to 50 for now and look to go 60 if I don't get a better replacement down the track. 

Is it worth doing anything with Basileus, or wait to see if I get anyone better?

Cheers for the advice

I can fully book Warmaiden but only got a few books for Spider. Worth going Warmaiden for now, Spider when I have the books or just go for Spider and book him as I go?

Cheers

Cheers, am very close to taking spider to 60. Weighing up fully booked Warmaiden over 1/4 booked Spider

Don't waste books on warmaiden, just farm dupes of her in campaign while leveling food.

The reason booking either warmaiden or spider is important is you need their decrease defense to be 100% chance to land, and the cooldown reduction.

That's what I've been doing. It took months to get my first Warmaiden, but then they came flooding in after that. even got 3 from 4 battles once. So is it worth taking Warmaiden to 60 first because I can fully book her and do Spider when I have the books?

Cheers

It's a tough decision. I think booking a aoe decrease defense champ is 100% necessary, so I would go with warmaiden for the time being, for a rare she is very strong. In time you will have multiple aoe decrease defense champs maxed out anyways. For the time being you could save the epic books for another champion or use warmaiden until your spider is ready.

Your account is pretty new, so I think it's best to get a full team of 60s asap and using warmaiden for the time being with save you a lot of epic books which are hard to come by.

My two cents. 

Still worth taking Spider to 60 I'd say. For arena, you'll want to keep using Warmaiden until Spider has the 100 percent chance and can take over her job, but in dungeons, he's arguably better already, even if only partially booked. And of course the thing with Warmaiden is you'd prefer to not have to take her to 60 at all, knowing she'll soon get benched and never used again except for FW. She should be able to do her job at 50 in most places, really. 

Basileus is worth taking to 60 eventually, sure, but I wouldn't invest precious lego books in him, nor prioritize him very much. 

Plarium allready announced a buff for Basileus in the next patch. The stunchance will increase to (booked) 100%, the damage multipliers of his skills "Majesty" and "Triple Puncture" will increase and the latter will ignore increase def buffs on the target in addition to the stuff it's allready ignoring. Probably Basileus will become a champ to max out then.

But yeah, if they kill Urogrim with a giant-nerf in the same patch, people will not be happy just because of Basileus.