You’ll get gems from story online and off line, side mission and side stories ( like ben and abe), achievements, for event going as far as possible every time is best way to go.
Spend them on the packs (10 in 1) an not the single hatches.
For story and almost all event ( except pvp and ) using a se monster with twin death sentence is good way to go. They can charge up bloodthirst that way. Others good se are the protectors, the 400 sec bomb and the one on one bloodcaver to some extend.
As for the epic the on with stun bomb are the best, the others I would recommend are stun revenge with create duplicate and the auto protect with sleep bomb.
As for the legendary almost all of them are good in some ways ( the weaker ones are about the got a buff soon)
I just wouldn’t recommend going for a full poison strat. They just can’t keep up since the monster in most event and story have increased stats. But don’t think they are weak, in PVP they are fearsome
Get the leorenix from the rookie mission, all the epic from the six trials and choco from elian side story.
As for event, the best for newcommers is the island challenge since you don’t use your monsters ( it’s easy to get to 120). Uc with just twin death sentence, 400 sec bomb, sleep bomb and little bit of luck you can got to 300 easily. Whale treasure isnt really kind to newcommer BUT you abuse the system and pick a friend summon. Get yourself some good ones, dragaia for earth, godfeather for thunder, wraithcaptain for water, etc
As soon as you unlock Online mode, go To Trainer’s dojo. The dojo missions teach you about powerful moves and synergy and give you much needed Hero Rank Exp and Youth Fruit. No monsters are needed for these battles as you will fight with premade teams and they don’t cost any online tickets either. You can access the dojo missions from the Island screen and from the Story missions tab.
The easiest way to get gems for beginners is to claim all the free gems from the online screen. This will give you about 25 gems. By completing a few Islands and doing side quests/Showdown of Old Corrolan will give you enough for 50 gems. 50 gems lets you open the 10 egg pack which will get you a bonus rare gem and hatch 10 monsters at once. The best time to open a 10 pack is during a Festival in which you get a guaranteed legendary (not all festivals have this feature so be vigilant!).