Drop Ghoul for Aries, SB for Vlad and if you have either an SK or DD: use them instead of Cupid. You can also try adding PK to your team. The idea is to disable opponents while you take them ASAP. Santa and Ghoul tend to feed the opponents energy, and they will proc if you don't take them out within the first 10 seconds.
I am L19 myself, my Heroes are SK, Gh, SB, TT, PD, all around level 160-180.
I would say that 75% of the enemy groups are boring: 4x elite 140 are not really a challenge, and I get those OFTEN.
20% are OK.
5% are too difficult and I die. (e.g. 9x Legy foes with L155)
As others have said you are using the wrong heroes. Another good option is to stick a revive/revite AC up front. I used that for a long time and could beat everything but the 15 stamina teams.
I have teams of either PD,Druid,Pixie,Cupid and Phantom King (lots of scatter in the team for the LR boss) or Aries, Skull Knight, Dread Drake, Valentina and PD which I use for taking altars. Either of these teams can flatten even the 15 stamina teams at lv23 Lost Realm in under 10 seconds.
Some crazy recommendations here based on stuff hilariously past your level of power.
Get everyone to 7/10 ASAP, and push your PD to 8/10 ASAP. As soon as you can, follow suit with everybody else, starting with ghoulem and cupid, and then get both PD and cupid to 9/10. Evo everybody, starting with ghoulem.
Put your PD and cupid in the front line so their proc gets triggered as fast as possible. TG works best in the front line too, but you'll have to experiment between him, cupid, and ghoulem to see which gives you the most bang for the buck in your front line. Leave PD there no matter what.
On lvl 22 and I also never lose. In fact was hoping that this was a post about getting more challenging scenarios in LR, lol!
I use SK, Pixie, PK, PD and Ghoul. I have scatter crests on SK and PK. If you don't have SK, use DD. I find that DD is the single most effect hero in LR except for demons. Ghoul is also a must since he is never stunned.