2)不告而别。"而"is a fuction word especially in old Chinese, so you can simply regard 不告而别 as a phrase. 3)乘虚而入。literally, getting in a room when the door is not closed because oft carelessness, usually the host would not like someone to get in. or to take advantage of others' difficulties.或者也可以简单理解为“占便宜”

1)跟他在公司打拼奋斗。 Struggling for work with him in the firm/company.yes, you are right. Just 打拼 or 奋斗 is enough. It is not for emphasis on 奋斗, and 打拼=奋斗 here. Like "There are monkeys and monkeys and monkeys on the monkey Island", Just for 强调。 2）我并不是不告而别。 不告而别=bundle off/ desert/ leave(go away)without say goodbye. 而别 is not a phrase, 而=when/while. 3)完全没办法乘虚而别。 It's wrong, I guess you want to express 乘虚而入，It's a idiom. Take a advantage of a weak point into. swoop in. get a chance to step in. 完全没办法乘虚而入=There is no way that you/sb can 乘虚而入。 4)你让我情何以堪哪? How should I get? Can stand? what could I do?

1) "跟"他在公司打拼奋斗。 if you put 打拼 and 奋斗 together, it just intensify the rhetoric, they both are noun. or you can just say 跟他在公司(努力的/努力地)工作 ,, 2) 我并不是不告而别。？ "別" means " leave "... so " 不告而別" means " leave without saying goodbye " 3）完全没办法乘虚而别。 ? the idiom should be 乘虛而 " 入 " ..... but I don't know how to explain in English.. sorry