I have checked the market lots of time and I think I have tried most towers and spell and what took my attention is Celene the hero which is the only available one and asked myself if it worth to be bought or rented from the market and what it will add to your formation in the battle as fiends coming in waves.
I have found that Hero is the best Card to be used not only for its legendary type but also for some good features that make it more special than spells and towers as it can be used without mana and this will make mana available for towers and spell and this make you play with extra item without spending your mana in the battle.
Other good thing about here as it can be used without mana so every wave you can get it back to deck and change its position and this is amazing feature and for me I think best use for Celene Hero is that it is a best gate defender as you can use it near the gate and keep it attacking fiends from melee and ranged attack.
It has a hit points that can reach up to 3000 and a very good attack range of 255 which make it so suitable for ranged attack type and if fiend gets close to it, its Melee attack type will work as it is 210 and its ranged physical attack is up to 600 so it is perfect in defending the gate as last line of defense when fiends come with low health after towers and spells attack.
As You can see it worth about 40 DEC to be rented for a day and this is very good as you can pass whatever levels you want for a whole day so it is a best legendary card that worth trying.